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Losses And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) $ in Thousands	Dec. 31, 2017 USD ($) number_of_claims	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Loss and allocated loss adjustment expense reserves, net of reinsurance	$ 1,359,898
Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	15,002,453
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	13,860,983
All outstanding liabilities before 2008, net of reinsurance	(169)
Loss and allocated loss adjustment expense reserves, net of reinsurance	1,141,300
Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,987,298
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,852,242
All outstanding liabilities before 2008, net of reinsurance	2,141
Loss and allocated loss adjustment expense reserves, net of reinsurance	137,195
Short-duration insurance contracts, accident year 2007 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,466,687	$ 1,466,137	$ 1,466,108	$ 1,465,623	$ 1,463,659	$ 1,461,084	$ 1,455,858	$ 1,442,691	$ 1,440,301	$ 1,505,732
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 55
Cumulative Number of Reported Claims | number_of_claims	199,223
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,466,704	1,465,832	1,464,763	1,463,384	1,459,216	1,450,172	1,418,274	1,345,354	1,226,787	992,844
Short-duration insurance contracts, accident year 2007 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	139,188	139,216	139,156	139,149	138,803	138,190	139,142	138,605	139,549	146,486
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | number_of_claims	16,497
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 138,963	138,922	138,809	138,680	138,137	137,284	135,500	129,821	122,239	$ 86,954
Short-duration insurance contracts, accident year 2008 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,371,812	1,372,259	1,372,694	1,371,779	1,365,551	1,361,652	1,361,116	1,349,025	1,372,833
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 82
Cumulative Number of Reported Claims | number_of_claims	185,624
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,371,933	1,371,956	1,370,564	1,363,526	1,355,210	1,326,439	1,260,424	1,137,807	913,340
Short-duration insurance contracts, accident year 2008 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	134,359	134,478	134,597	134,718	133,121	133,087	131,680	135,000	135,889
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 26
Cumulative Number of Reported Claims | number_of_claims	17,411
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 134,229	134,301	134,378	134,381	132,180	130,928	126,591	119,306	$ 86,034
Short-duration insurance contracts, accident year 2009 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,388,077	1,386,105	1,381,056	1,379,336	1,374,638	1,364,307	1,357,750	1,367,547
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 158
Cumulative Number of Reported Claims | number_of_claims	184,135
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,387,835	1,384,333	1,375,799	1,365,464	1,335,871	1,268,142	1,143,984	908,954
Short-duration insurance contracts, accident year 2009 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	159,662	159,608	160,015	160,206	160,472	160,983	157,566	165,727
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 117
Cumulative Number of Reported Claims | number_of_claims	20,776
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 159,268	158,943	158,053	156,853	155,191	149,084	137,628	$ 95,057
Short-duration insurance contracts, accident year 2010 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,405,112	1,398,518	1,393,878	1,388,363	1,380,557	1,367,000	1,343,919
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 9,367
Cumulative Number of Reported Claims | number_of_claims	180,857
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,394,684	1,391,101	1,378,920	1,347,082	1,277,808	1,152,459	926,983
Short-duration insurance contracts, accident year 2010 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	170,126	169,621	169,390	169,600	170,052	170,623	167,414
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 234
Cumulative Number of Reported Claims | number_of_claims	23,070
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 168,914	168,621	167,806	166,375	162,870	153,845	$ 111,909
Short-duration insurance contracts, accident year 2011 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,436,034	1,426,735	1,414,878	1,409,104	1,408,222	1,424,754
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 1,756
Cumulative Number of Reported Claims | number_of_claims	180,654
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,422,705	1,409,911	1,372,828	1,304,511	1,194,648	955,647
Short-duration insurance contracts, accident year 2011 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	191,804	192,057	191,548	190,376	188,010	196,063
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 428
Cumulative Number of Reported Claims | number_of_claims	24,621
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 190,649	190,373	188,121	182,756	175,029	$ 128,618
Short-duration insurance contracts, accident year 2012 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,464,071	1,458,421	1,447,881	1,431,058	1,448,567
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 4,011
Cumulative Number of Reported Claims | number_of_claims	184,561
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,447,004	1,413,999	1,340,724	1,217,906	974,445
Short-duration insurance contracts, accident year 2012 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	187,165	186,795	188,026	188,915	191,903
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 524
Cumulative Number of Reported Claims | number_of_claims	23,436
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 185,168	183,860	180,858	174,295	$ 133,528
Short-duration insurance contracts, accident year 2013 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,486,322	1,473,545	1,454,366	1,467,175
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 12,959
Cumulative Number of Reported Claims | number_of_claims	180,410
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,432,472	1,358,472	1,231,413	967,481
Short-duration insurance contracts, accident year 2013 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	202,513	203,218	202,621	199,298
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 1,330
Cumulative Number of Reported Claims | number_of_claims	24,826
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 198,758	194,605	186,996	$ 139,615
Short-duration insurance contracts, accident year 2014 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,610,839	1,588,443	1,551,105
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 40,444
Cumulative Number of Reported Claims | number_of_claims	169,997
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,466,368	1,336,223	1,040,253
Short-duration insurance contracts, accident year 2014 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	233,501	234,881	234,800
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 4,368
Cumulative Number of Reported Claims | number_of_claims	23,955
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 224,178	213,994	$ 163,196
Short-duration insurance contracts, accident year 2015 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,669,642	1,672,853
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 102,672
Cumulative Number of Reported Claims | number_of_claims	153,776
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,395,199	1,094,006
Short-duration insurance contracts, accident year 2015 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	259,489	250,691
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 9,995
Cumulative Number of Reported Claims | number_of_claims	24,254
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 234,215	$ 173,537
Short-duration insurance contracts, accident year 2016 [Member] | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,703,857
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 347,296
Cumulative Number of Reported Claims | number_of_claims	140,219
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,076,079
Short-duration insurance contracts, accident year 2016 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	309,491
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 47,502
Cumulative Number of Reported Claims | number_of_claims	28,182
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 217,900

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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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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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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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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.

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