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Reserve for Loss and Loss Expense (Claims Development) (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Net outstanding liabilities	$ 12,185,164
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	8,973,347
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,576,212
Reserve for loss and loss expense	4,260,355	$ 4,067,163
Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	364,395
Short-duration Insurance Contracts, Accident Year 2011 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,009,162	1,013,175	$ 1,013,115	$ 1,019,351	$ 1,023,726	$ 1,033,518	$ 1,056,107	$ 1,062,233	$ 1,061,667	$ 1,042,576
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 36,641
Short-duration Insurance Contract, Cumulative Number of Reported Claims	105,039
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 957,391	950,836	940,626	924,111	901,801	852,202	782,655	692,730	585,867	391,944
Short-duration Insurance Contracts, Accident Year 2012 [Member]
Claims Development [Line Items]
Net outstanding liabilities	962,091	968,536	973,411	973,644	973,089	998,028	1,020,655	1,071,290	1,065,437
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41,426
Short-duration Insurance Contract, Cumulative Number of Reported Claims	104,343
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 905,816	898,269	879,372	856,195	810,135	743,742	651,544	555,819	378,067
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Claims Development [Line Items]
Net outstanding liabilities	973,739	984,858	987,763	1,002,316	1,021,007	1,047,230	1,062,045	1,044,142
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 57,866
Short-duration Insurance Contract, Cumulative Number of Reported Claims	91,539
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 904,825	891,841	872,331	833,823	748,758	644,475	518,872	335,956
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,089,529	1,100,208	1,104,218	1,124,014	1,146,990	1,133,798	1,107,513
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 60,028
Short-duration Insurance Contract, Cumulative Number of Reported Claims	95,366
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 1,002,157	981,868	936,425	855,959	736,154	614,075	405,898
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,108,860	1,119,441	1,138,313	1,144,830	1,130,513	1,114,081
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 77,855
Short-duration Insurance Contract, Cumulative Number of Reported Claims	94,594
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 967,857	929,222	845,868	725,385	581,203	376,641
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,180,829	1,199,734	1,227,142	1,203,634	1,188,608
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 126,935
Short-duration Insurance Contract, Cumulative Number of Reported Claims	95,203
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 983,852	892,390	764,331	617,958	387,272
Short-duration Insurance Contracts, Accident Year 2017 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,288,526	1,313,585	1,313,372	1,270,110
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 206,627
Short-duration Insurance Contract, Cumulative Number of Reported Claims	98,918
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 954,792	829,134	678,453	433,440
Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,457,415	1,461,603	1,413,800
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 342,256
Short-duration Insurance Contract, Cumulative Number of Reported Claims	105,772
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 942,893	779,466	511,271
Short-Duration Insurance Contract, Accident Year 2019 [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,523,041	1,483,945
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 539,113
Short-duration Insurance Contract, Cumulative Number of Reported Claims	101,631
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 781,462	510,091
Short-Duration Insurance Contract, Accident Year 2020
Claims Development [Line Items]
Net outstanding liabilities	1,591,972
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 820,762
Short-duration Insurance Contract, Cumulative Number of Reported Claims	85,549
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 572,302
Standard Commercial Lines [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	2,633,440
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,347,996
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,380,902
Standard Commercial Lines [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,716,956
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,132,576
Liability for Unpaid Claims and Claims Adjustment Expense, Net	828,146
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	2,781,151
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,154,487
Liability for Unpaid Claims and Claims Adjustment Expense, Net	630,512
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	546,289
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	462,643
Liability for Unpaid Claims and Claims Adjustment Expense, Net	91,419
Standard Commercial Lines [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,466,423
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,392,416
Liability for Unpaid Claims and Claims Adjustment Expense, Net	74,182
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	95,458
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	243,766
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,848
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	7,773
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	175
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	209,846	213,485	211,500	212,011	211,196	217,256	230,785	239,480	228,720	227,769
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13,501
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,691
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 190,650	187,276	181,856	170,525	159,768	135,377	102,978	73,643	42,692	13,924
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	191,875	192,863	194,821	200,674	205,708	210,591	215,114	214,743	218,973	205,238
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 21,900
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,863
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 163,554	161,696	158,535	154,320	149,269	139,232	134,646	118,847	90,836	42,941
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	172,684	173,401	173,507	174,514	174,882	172,617	178,421	182,325	183,044	174,006
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 205
Short-duration Insurance Contract, Cumulative Number of Reported Claims	25,651
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 172,413	172,365	170,913	170,000	166,082	157,291	142,507	121,576	99,196	69,849
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	58,697	58,948	58,735	58,456	58,966	59,256	58,242	51,047	57,083	54,469
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 516
Short-duration Insurance Contract, Cumulative Number of Reported Claims	4,961
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 57,385	57,380	57,365	57,045	55,856	52,114	46,444	41,011	37,362	27,884
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	130,994	131,002	131,009	131,049	131,113	131,353	131,282	130,942	131,667	136,954
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9
Short-duration Insurance Contract, Cumulative Number of Reported Claims	9,038
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 131,083	131,092	131,100	131,089	131,115	131,060	130,681	129,579	127,580	94,538
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	178,285	182,085	180,659	175,268	175,305	194,144	215,083	245,561	238,979
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15,055
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,034
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 158,397	156,186	144,451	130,866	109,448	89,008	56,580	35,241	13,030
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	177,658	178,774	183,314	187,359	188,596	195,197	199,360	208,036	203,864
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 23,181
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,624
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 148,750	146,739	143,281	139,477	132,052	122,755	108,211	86,909	40,911
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	184,477	185,357	184,633	186,128	184,367	184,289	183,527	191,947	179,551
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 811
Short-duration Insurance Contract, Cumulative Number of Reported Claims	24,295
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 183,098	181,353	179,501	176,656	168,114	148,669	127,235	105,371	73,316
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	43,444	43,717	43,418	43,747	44,077	44,172	46,303	48,029	54,342
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 117
Short-duration Insurance Contract, Cumulative Number of Reported Claims	5,545
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 42,075	41,356	41,326	40,627	38,766	37,215	35,089	31,833	22,199
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	117,200	117,220	117,225	117,170	117,102	116,658	115,375	114,224	118,464
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6
Short-duration Insurance Contract, Cumulative Number of Reported Claims	8,517
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 116,673	116,674	116,671	116,625	116,291	114,699	111,503	108,834	81,528
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	195,697	202,697	203,831	210,785	225,709	239,776	251,421	250,609
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 19,110
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,397
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 169,847	163,764	153,628	139,114	104,587	72,127	35,113	12,789
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	157,645	159,767	162,787	166,662	173,160	187,658	194,318	199,794
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 23,321
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,382
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 129,224	126,822	124,130	118,669	109,739	96,376	74,568	36,829
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	208,040	209,602	207,975	210,410	207,994	209,197	205,282	188,289
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,694
Short-duration Insurance Contract, Cumulative Number of Reported Claims	25,886
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 205,064	202,622	200,750	189,626	169,850	140,015	109,893	76,469
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	39,358	39,699	39,709	41,369	40,624	41,005	42,618	49,617
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 554
Short-duration Insurance Contract, Cumulative Number of Reported Claims	3,483
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 39,212	39,085	38,464	37,993	34,760	30,845	26,592	17,412
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	90,486	90,218	90,278	90,436	90,005	90,103	90,639	88,101
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 11
Short-duration Insurance Contract, Cumulative Number of Reported Claims	5,715
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 90,917	90,646	90,696	90,840	90,350	90,446	87,874	60,244
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	229,679	237,125	234,082	239,333	257,132	249,946	244,312
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29,018
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,652
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 187,352	179,192	154,957	121,969	79,972	46,825	14,901
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	159,604	160,646	164,420	172,515	182,579	187,065	199,346
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25,582
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,495
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 127,858	124,077	119,392	113,626	100,876	78,944	35,924
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	216,461	217,334	218,172	219,925	216,824	212,725	200,534
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,096
Short-duration Insurance Contract, Cumulative Number of Reported Claims	27,896
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 212,481	209,655	202,821	180,701	148,884	117,169	80,810
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	57,302	58,093	58,517	59,806	62,548	60,949	55,962
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 616
Short-duration Insurance Contract, Cumulative Number of Reported Claims	4,066
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 55,708	55,458	52,940	49,460	44,911	40,584	28,914
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	136,237	136,212	138,155	138,751	136,820	136,249	141,192
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,516
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 135,928	136,008	137,418	137,883	136,634	132,909	101,131
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	214,176	219,204	233,249	246,990	245,710	254,720
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 38,182
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,537
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 157,071	144,216	116,804	78,668	39,978	14,665
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	172,572	176,242	179,642	183,604	194,639	193,729
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22,368
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,551
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 124,046	120,097	112,601	98,195	77,320	33,857
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	261,386	260,565	259,495	253,074	240,958	220,994
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,729
Short-duration Insurance Contract, Cumulative Number of Reported Claims	29,590
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 249,905	238,142	211,515	175,866	132,260	91,347
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	52,325	54,454	55,925	57,245	53,768	52,871
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,178
Short-duration Insurance Contract, Cumulative Number of Reported Claims	3,963
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 49,839	49,073	46,571	42,710	36,014	24,189
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	112,582	112,496	111,566	111,750	109,513	110,270
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 24
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,406
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 112,117	110,969	110,994	109,829	106,182	79,048
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	252,733	263,245	277,986	272,048	277,214
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 61,329
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,753
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 164,136	133,757	89,431	46,549	15,684
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	156,540	166,009	176,248	184,946	196,774
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29,220
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,582
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 115,159	109,166	98,037	78,531	34,525
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	290,359	285,304	285,302	274,367	255,187
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 8,469
Short-duration Insurance Contract, Cumulative Number of Reported Claims	31,465
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 264,858	233,939	200,701	155,720	106,022
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	53,367	53,835	54,993	53,792	52,335
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,877
Short-duration Insurance Contract, Cumulative Number of Reported Claims	3,847
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 48,786	45,308	39,973	36,848	24,655
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	123,567	124,487	125,937	126,185	121,927
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 60
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,741
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 123,601	123,828	122,930	118,789	83,966
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	289,883	301,384	293,128	293,747
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 114,160
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,096
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 131,980	92,355	49,470	17,366
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	162,672	175,853	184,306	195,202
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 34,474
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,808
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 110,645	100,645	82,216	40,375
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	322,197	324,291	329,389	301,274
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22,447
Short-duration Insurance Contract, Cumulative Number of Reported Claims	32,775
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 262,349	220,422	178,823	117,287
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	48,067	51,524	48,698	46,624
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,981
Short-duration Insurance Contract, Cumulative Number of Reported Claims	3,885
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,359	36,950	31,337	21,865
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	153,664	149,044	149,106	138,773
Short-duration Insurance, Contracts Incurred But Not Reported (IBNR) Claims Liability, Net	$ (289)
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,900
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 152,018	148,589	142,338	99,047
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	345,224	336,326	317,934
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 182,931
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,350
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 108,421	60,784	19,531
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	181,151	193,818	193,894
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 48,883
Short-duration Insurance Contract, Cumulative Number of Reported Claims	11,111
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 105,903	84,780	41,122
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	345,547	352,487	347,908
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47,298
Short-duration Insurance Contract, Cumulative Number of Reported Claims	35,418
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 243,713	193,788	134,867
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	62,427	57,202	55,024
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 12,432
Short-duration Insurance Contract, Cumulative Number of Reported Claims	4,229
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 44,316	39,791	29,995
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	192,558	190,834	183,177
Short-duration Insurance, Contracts Incurred But Not Reported (IBNR) Claims Liability, Net	$ (841)
Short-duration Insurance Contract, Cumulative Number of Reported Claims	8,280
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 192,698	184,813	135,416
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	356,363	347,150
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 248,289
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,531
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 58,284	18,097
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	188,596	188,625
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 69,909
Short-duration Insurance Contract, Cumulative Number of Reported Claims	10,267
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 77,878	37,826
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	398,346	385,212
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 111,327
Short-duration Insurance Contract, Cumulative Number of Reported Claims	35,703
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 221,590	149,538
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	59,466	53,531
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 12,872
Short-duration Insurance Contract, Cumulative Number of Reported Claims	3,541
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 41,587	27,718
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	177,075	173,826
Short-duration Insurance, Contracts Incurred But Not Reported (IBNR) Claims Liability, Net	$ (738)
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,262
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 172,768	130,891
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	361,554
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 311,657
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,990
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 21,858
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	168,643
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 97,510
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,017
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 29,559
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	381,654
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 195,279
Short-duration Insurance Contract, Cumulative Number of Reported Claims	28,366
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 139,016
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Business Owners' Policies [Member]
Claims Development [Line Items]
Net outstanding liabilities	71,836
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 17,392
Short-duration Insurance Contract, Cumulative Number of Reported Claims	5,019
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 43,376
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Commercial Property [Member]
Claims Development [Line Items]
Net outstanding liabilities	232,060
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 37,050
Short-duration Insurance Contract, Cumulative Number of Reported Claims	9,494
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 164,613
Standard Personal Lines | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	1,063,316
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	975,981
Liability for Unpaid Claims and Claims Adjustment Expense, Net	94,237
Standard Personal Lines | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	794,625
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	755,451
Liability for Unpaid Claims and Claims Adjustment Expense, Net	44,159
Standard Personal Lines | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	6,902
Standard Personal Lines | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | Homeowners [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	4,985
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	114,038	114,056	113,921	113,988	113,830	114,241	112,993	113,686	116,164	113,232
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 31
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,700
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 113,996	113,856	113,664	113,551	112,732	111,085	105,068	93,878	82,102	61,323
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	94,524	94,572	94,587	94,378	94,183	94,543	94,167	97,761	98,211	103,804
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 37
Short-duration Insurance Contract, Cumulative Number of Reported Claims	15,111
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 94,452	94,458	94,412	94,007	93,720	93,312	92,185	91,718	89,963	71,668
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	109,634	109,701	109,795	110,300	110,294	109,324	109,832	114,921	113,771
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 78
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,333
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 109,554	109,482	109,447	109,355	107,890	102,977	94,842	82,729	63,704
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	86,519	86,567	86,483	86,330	86,271	86,667	86,560	82,744	87,260
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 44
Short-duration Insurance Contract, Cumulative Number of Reported Claims	16,942
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 85,899	85,897	85,642	85,562	85,196	84,250	82,720	79,584	69,056
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	107,803	107,916	107,680	107,759	106,703	106,225	109,620	108,417
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 121
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,376
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 107,419	106,876	106,679	105,131	100,528	92,637	80,861	61,384
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	71,948	72,318	72,148	71,714	72,145	71,494	72,528	73,670
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,749
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 72,446	72,433	72,197	71,776	69,775	67,838	65,528	50,664
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	107,106	107,104	106,821	107,452	106,757	109,325	102,250
Short-duration Insurance, Contracts Incurred But Not Reported (IBNR) Claims Liability, Net	$ (42)
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,508
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 106,478	105,709	104,055	99,173	92,589	83,739	62,519
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	83,525	83,824	83,539	83,844	83,637	82,461	80,111
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 442
Short-duration Insurance Contract, Cumulative Number of Reported Claims	8,773
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 82,831	82,836	82,583	81,664	79,751	76,007	61,561
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	98,588	98,718	99,570	100,214	99,698	96,387
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 295
Short-duration Insurance Contract, Cumulative Number of Reported Claims	20,865
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 97,275	95,997	92,102	87,163	76,470	58,725
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	74,673	74,978	74,723	76,559	76,400	76,637
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 483
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,750
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 74,052	74,079	72,927	72,202	70,078	52,589
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	99,544	101,140	100,202	98,032	92,727
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 452
Short-duration Insurance Contract, Cumulative Number of Reported Claims	19,824
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 98,080	94,372	86,752	76,823	57,961
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	61,735	61,723	62,391	60,931	60,105
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 442
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,892
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 61,187	60,082	59,546	57,333	42,252
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	103,260	103,653	105,139	101,880
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,607
Short-duration Insurance Contract, Cumulative Number of Reported Claims	20,739
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 97,628	91,479	82,730	62,854
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	70,064	70,365	67,978	59,167
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,822
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,385
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 67,767	67,193	63,290	45,466
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	112,030	113,569	111,594
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,597
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,668
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 99,982	89,628	69,721
Standard Personal Lines | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	69,832	68,526	62,961
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,129
Short-duration Insurance Contract, Cumulative Number of Reported Claims	7,596
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 65,348	64,137	49,430
Standard Personal Lines | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	115,688	114,043
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13,260
Short-duration Insurance Contract, Cumulative Number of Reported Claims	22,804
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 92,162	69,699
Standard Personal Lines | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	72,772	64,306
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,765
Short-duration Insurance Contract, Cumulative Number of Reported Claims	6,970
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 67,631	49,680
Standard Personal Lines | Short-Duration Insurance Contract, Accident Year 2020 | Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	95,625
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 30,333
Short-duration Insurance Contract, Cumulative Number of Reported Claims	16,309
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 53,407
Standard Personal Lines | Short-Duration Insurance Contract, Accident Year 2020 | Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	109,033
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 16,218
Short-duration Insurance Contract, Cumulative Number of Reported Claims	9,003
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 83,838
E&S Lines [Member]
Claims Development [Line Items]
Reserve for loss and loss expense									15,000
E&S Lines [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	816,172
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	426,833
Liability for Unpaid Claims and Claims Adjustment Expense, Net	389,460
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	121
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	13,344	11,554	12,119	10,228	9,652	10,273	12,207	9,853	7,102	8,127
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 408
Short-duration Insurance Contract, Cumulative Number of Reported Claims	1,337
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 10,744	9,604	9,819	10,256	9,912	9,954	6,445	3,200	806	$ 0
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	44,348	44,622	46,444	45,988	46,165	46,149	43,175	42,621	42,367
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,126
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,055
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,615	39,832	38,298	36,278	32,343	25,064	16,430	7,914	$ 3,722
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	68,029	68,451	68,972	67,647	69,112	67,099	60,309	55,468
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13,032
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,299
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 55,988	54,974	51,142	46,108	35,200	21,980	9,470	$ 2,715
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	70,846	71,153	71,206	71,719	69,929	63,505	55,316
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,352
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,101
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 64,698	60,092	53,780	43,877	25,571	12,234	$ 2,353
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	90,126	90,355	90,488	82,404	76,432	75,498
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 12,518
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,844
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 71,421	64,529	50,712	29,389	13,057	$ 3,036
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	104,730	105,120	104,655	96,416	94,451
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25,507
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,925
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 69,448	56,581	33,950	16,195	$ 3,720
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	99,395	99,866	95,783	91,438
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 26,778
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,727
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 53,238	34,179	14,672	$ 5,057
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	103,184	103,004	98,324
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41,204
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,637
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 39,174	21,337	$ 5,509
E&S Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	118,298	117,087
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 78,482
Short-duration Insurance Contract, Cumulative Number of Reported Claims	2,278
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 17,812	$ 4,422
E&S Lines [Member] | Short-Duration Insurance Contract, Accident Year 2020 | General Liability [Member]
Claims Development [Line Items]
Net outstanding liabilities	103,872
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 90,970
Short-duration Insurance Contract, Cumulative Number of Reported Claims	1,138
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 3,695

X

- Definition

Undiscounted amount after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ClaimsDevelopmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to casualty insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date.

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- Definition

Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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- Definition

Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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- Definition

Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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Cumulative number of reported claims for short-duration insurance contracts.

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