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Losses And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands	Dec. 31, 2016 USD ($) number_of_claims	Dec. 31, 2015 USD ($)	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 ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Loss and allocated loss adjustment expense reserves, net of reinsurance	$ 1,196,434
Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	14,879,081
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	13,862,037
All outstanding liabilities before 2007, net of reinsurance	(212)
Loss and allocated loss adjustment expense reserves, net of reinsurance	1,016,832
Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,780,518
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,667,129
All outstanding liabilities before 2007, net of reinsurance	2,661
Loss and allocated loss adjustment expense reserves, net of reinsurance	116,050
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	1,636,065	$ 1,636,379	$ 1,635,529	$ 1,634,127	$ 1,633,169	$ 1,633,606	$ 1,624,875	$ 1,614,648	$ 1,610,874	$ 1,591,217
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 52
Cumulative Number of Reported Claims | number_of_claims	230
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,636,204	1,635,631	1,634,888	1,633,178	1,628,413	1,615,964	1,581,001	1,496,543	1,365,854	1,040,933
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	110,204	110,142	110,161	110,092	110,052	109,836	111,378	112,042	116,950	119,505
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 42
Cumulative Number of Reported Claims | number_of_claims	14
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 110,075	110,064	109,949	109,932	109,874	109,305	108,404	104,340	100,774	$ 71,165
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	1,466,137	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	$ 82
Cumulative Number of Reported Claims | number_of_claims	199
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2008 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ 5
Cumulative Number of Reported Claims | number_of_claims	16
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 138,922	138,809	138,680	138,137	137,284	135,500	129,821	122,239	$ 86,954
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	1,372,259	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	$ 166
Cumulative Number of Reported Claims | number_of_claims	186
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2009 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ (3)
Cumulative Number of Reported Claims | number_of_claims	17
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 134,301	134,378	134,381	132,180	130,928	126,591	119,306	$ 86,034
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	1,386,105	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	$ 390
Cumulative Number of Reported Claims | number_of_claims	184
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2010 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ (17)
Cumulative Number of Reported Claims | number_of_claims	21
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 158,943	158,053	156,853	155,191	149,084	137,628	$ 95,057
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	1,398,518	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	$ 4,069
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,391,101	1,378,920	1,347,082	1,277,808	1,152,459	926,983
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	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	$ 325
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 168,621	167,806	166,375	162,870	153,845	$ 111,909
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	1,426,735	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	$ 4,531
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,409,911	1,372,828	1,304,511	1,194,648	955,647
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	192,057	191,548	190,376	188,010	196,063
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 585
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 190,373	188,121	182,756	175,029	$ 128,618
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	1,458,421	1,447,881	1,431,058	1,448,567
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 10,873
Cumulative Number of Reported Claims | number_of_claims	185
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,413,999	1,340,724	1,217,906	974,445
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	186,795	188,026	188,915	191,903
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 775
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 183,860	180,858	174,295	$ 133,528
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	1,473,545	1,454,366	1,467,175
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 32,532
Cumulative Number of Reported Claims | number_of_claims	180
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,358,472	1,231,413	967,481
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	203,218	202,621	199,298
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 2,993
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 194,605	186,996	$ 139,615
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	1,588,443	1,551,105
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 90,010
Cumulative Number of Reported Claims | number_of_claims	169
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,336,223	1,040,253
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	234,881	234,800
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 9,431
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 213,994	$ 163,196
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	1,672,853
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 303,353
Cumulative Number of Reported Claims | number_of_claims	145
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,094,006
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	250,440
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 36,538
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 173,435

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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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