Loss And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands	Dec. 31, 2020 USD ($) number_of_claims	Dec. 31, 2019 USD ($)	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	$ 1,820,077
Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	16,138,435
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	14,695,210
All outstanding liabilities before 2011, net of reinsurance	(531)
Net outstanding liabilities	1,442,694
Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,647,227
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,415,495
All outstanding liabilities before 2011, net of reinsurance	1,457
Net outstanding liabilities	233,189
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	1,401,543	$ 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	$ 59
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,401,592	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 2011 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	170,402	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	$ 167
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 169,958	169,899	169,757	168,914	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	1,440,537	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	$ 116
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,440,482	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 2012 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	193,002	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	$ 120
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 192,454	192,362	191,660	190,649	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	1,468,747	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	$ 816
Cumulative Number of Reported Claims | number_of_claims	185
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,466,356	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 2013 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	187,683	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	$ 142
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 186,816	186,494	186,132	185,168	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	1,501,713	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	$ 718
Cumulative Number of Reported Claims | number_of_claims	180
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,496,814	1,490,366	1,476,944	1,432,472	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	208,743	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	$ 592
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 207,524	203,333	202,193	198,758	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	1,649,170	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	$ 4,577
Cumulative Number of Reported Claims | number_of_claims	170
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,634,780	1,614,188	1,560,480	1,466,368	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	239,941	238,652	236,855	233,501	234,881	234,800
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 2,950
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 235,971	234,683	230,480	224,178	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	1,732,410	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	$ 8,206
Cumulative Number of Reported Claims | number_of_claims	155
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,699,069	1,656,192	1,554,217	1,395,199	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	260,496	259,708	259,497	259,489	250,691
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 1,821
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 256,642	253,919	245,878	234,215	$ 173,537
Short-duration Insurance Contracts, Accident Year 2017 | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,733,425	1,741,825	1,727,277	1,703,857
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 22,626
Cumulative Number of Reported Claims | number_of_claims	149
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,648,328	1,561,850	1,399,202	1,076,079
Short-duration Insurance Contracts, Accident Year 2017 | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	289,869	288,322	295,163	309,491
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 2,702
Cumulative Number of Reported Claims | number_of_claims	30
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 283,311	278,341	269,254	$ 217,900
Short-duration Insurance Contracts, Accident Year 2018 | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,785,071	1,773,502	1,781,817
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 60,829
Cumulative Number of Reported Claims | number_of_claims	146
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,588,049	1,417,637	1,082,127
Short-duration Insurance Contracts, Accident Year 2018 | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	310,695	308,361	311,798
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 9,704
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 286,658	271,534	$ 213,038
Short-Duration Insurance Contract, Accident Year 2019 | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,911,268	1,916,269
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 165,108
Cumulative Number of Reported Claims | number_of_claims	147
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,494,342	1,134,859
Short-Duration Insurance Contract, Accident Year 2019 | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	366,139	359,643
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 17,580
Cumulative Number of Reported Claims | number_of_claims	30
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 324,953	$ 240,240
Short-Duration Insurance Contract, Accident Year 2020 | Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	1,514,551
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 407,969
Cumulative Number of Reported Claims | number_of_claims	85
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 825,398
Short-Duration Insurance Contract, Accident Year 2020 | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	420,257
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 77,396
Cumulative Number of Reported Claims | number_of_claims	26
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 271,208