Loss And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands	Dec. 31, 2019 USD ($) number_of_claims	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Claims Development [Line Items]
Net outstanding liabilities	$ 1,745,170
Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	16,007,752
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	14,567,335
All outstanding liabilities before 2010, net of reinsurance	(557)
Net outstanding liabilities	1,439,861
Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,372,452
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,190,296
All outstanding liabilities before 2010, net of reinsurance	(576)
Net outstanding liabilities	182,731
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	1,388,086	$ 1,388,055	$ 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	$ 43
Cumulative Number of Reported Claims | number_of_claims	184
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,388,322	1,388,140	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 2007 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	159,652	159,720	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	$ 1
Cumulative Number of Reported Claims | number_of_claims	21
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 159,491	159,435	159,268	158,943	158,053	156,853	155,191	149,084	137,628	$ 95,057
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	1,401,151	1,401,178	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	$ 73
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,400,958	1,400,441	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 2008 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	170,174	170,334	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	$ 75
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 169,899	169,757	168,914	168,621	167,806	166,375	162,870	153,845	$ 111,909
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	1,439,660	1,438,250	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	$ 493
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,438,686	1,434,956	1,422,705	1,409,911	1,372,828	1,304,511	1,194,648	955,647
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	192,790	192,905	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	$ 6
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 192,362	191,660	190,649	190,373	188,121	182,756	175,029	$ 128,618
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	1,468,237	1,468,294	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	$ 1,149
Cumulative Number of Reported Claims | number_of_claims	185
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,464,277	1,460,352	1,447,004	1,413,999	1,340,724	1,217,906	974,445
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	187,147	188,014	187,165	186,795	188,026	188,915	191,903
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 42
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 186,494	186,132	185,168	183,860	180,858	174,295	$ 133,528
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	1,501,075	1,498,504	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	$ 3,469
Cumulative Number of Reported Claims | number_of_claims	180
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,490,366	1,476,944	1,432,472	1,358,472	1,231,413	967,481
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	208,003	204,986	202,513	203,218	202,621	199,298
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 3,259
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 203,333	202,193	198,758	194,605	186,996	$ 139,615
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	1,645,950	1,634,435	1,610,839	1,588,443	1,551,105
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 9,132
Cumulative Number of Reported Claims | number_of_claims	170
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,614,188	1,560,480	1,466,368	1,336,223	1,040,253
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	238,652	236,855	233,501	234,881	234,800
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 1,978
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 234,683	230,480	224,178	213,994	$ 163,196
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	1,731,997	1,713,696	1,669,642	1,672,853
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 22,097
Cumulative Number of Reported Claims | number_of_claims	154
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,656,192	1,554,217	1,395,199	1,094,006
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	259,708	259,497	259,489	250,691
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 2,092
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 253,919	245,878	234,215	$ 173,537
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	1,741,825	1,727,277	1,703,857
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 58,648
Cumulative Number of Reported Claims | number_of_claims	149
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,561,850	1,399,202	1,076,079
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	288,322	295,163	309,491
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 4,310
Cumulative Number of Reported Claims | number_of_claims	30
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 278,341	269,254	$ 217,900
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	1,773,502	1,781,817
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 139,022
Cumulative Number of Reported Claims | number_of_claims	146
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,417,637	1,082,127
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	308,361	311,798
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 15,686
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 271,534	$ 213,038
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	1,916,269
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 438,858
Cumulative Number of Reported Claims | number_of_claims	140
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,134,859
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	359,643
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 64,136
Cumulative Number of Reported Claims | number_of_claims	28
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 240,240