Unpaid Claims and Claim Expenses - Incurred and Paid Claims by Accident Year on a Net Basis (Details) $ in Thousands	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Homeowners
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 1,270,282
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	1,214,781
Outstanding prior to 2011	32
Prior years paid
Liabilities for claims and claim adjustment expenses, net of reinsurance	55,533
Homeowners | 2011
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	148,067	$ 148,067	$ 148,079	$ 148,370	$ 148,414	$ 148,755	$ 148,860	$ 150,791	$ 150,334	$ 150,141
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	29,532,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 148,067	148,067	148,069	148,014	148,026	147,451	146,908	145,852	142,846	123,046
Homeowners | 2012
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	106,032	106,028	106,000	106,348	106,308	106,486	109,360	109,156	108,754
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	21,578,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 106,025	106,021	105,993	105,909	105,561	105,156	104,203	101,566	84,260
Homeowners | 2013
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	101,711	101,709	101,769	102,345	102,407	103,982	107,489	105,584
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	19,222,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 101,711	101,709	101,677	101,527	100,968	99,361	96,599	76,890
Homeowners | 2014
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	106,414	106,458	106,554	106,844	109,059	113,505	111,647
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	20,084,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 106,419	106,388	106,258	106,081	105,704	103,030	83,314
Homeowners | 2015
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	114,942	115,050	114,053	114,404	115,134	111,706
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 67
Cumulative Number of Reported Claims | claim	18,716,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 114,861	114,648	113,321	111,882	109,303	90,704
Homeowners | 2016
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	117,940	117,933	117,009	118,604	115,931
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 241
Cumulative Number of Reported Claims | claim	19,860,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 117,688	117,537	115,053	113,186	95,772
Homeowners | 2017
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	130,693	132,666	129,818	126,285
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 585
Cumulative Number of Reported Claims | claim	19,850,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 130,017	129,767	128,518	106,800
Homeowners | 2018
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	158,861	157,404	166,793
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,352
Cumulative Number of Reported Claims | claim	21,037,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 157,004	152,356	130,548
Homeowners | 2019
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	129,901	130,391
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 1,949
Cumulative Number of Reported Claims | claim	17,463,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 126,208	103,790
Homeowners | 2020
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	155,721
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 32,267
Cumulative Number of Reported Claims | claim	18,229,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 106,781
Automobile liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	1,659,290
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	1,480,265
Outstanding prior to 2011	724
Prior years paid
Liabilities for claims and claim adjustment expenses, net of reinsurance	179,749
Automobile liability | 2011
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	139,530	138,849	138,949	138,891	139,840	142,488	143,133	145,735	146,713	150,803
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 90
Cumulative Number of Reported Claims | claim	46,163,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 138,708	138,692	138,689	138,358	138,151	136,906	133,931	126,812	108,837	61,070
Automobile liability | 2012
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	145,946	145,515	145,620	145,847	147,594	149,346	150,336	153,815	156,448
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 233
Cumulative Number of Reported Claims | claim	46,008,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 145,256	145,184	145,121	144,622	142,916	138,641	127,185	109,574	61,279
Automobile liability | 2013
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	148,288	148,135	147,993	148,111	150,657	150,720	152,858	153,860
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 668
Cumulative Number of Reported Claims | claim	47,336,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 147,443	147,409	146,770	145,291	139,954	131,214	108,856	62,224
Automobile liability | 2014
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	159,263	159,355	159,794	159,937	158,470	157,249	155,105
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 62
Cumulative Number of Reported Claims | claim	49,347,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 158,644	157,596	155,758	149,059	139,463	117,468	61,329
Automobile liability | 2015
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	179,186	178,654	178,325	177,021	172,553	165,517
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 828
Cumulative Number of Reported Claims | claim	50,579,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 176,728	174,495	170,088	157,980	134,473	70,836
Automobile liability | 2016
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	188,079	186,568	184,567	184,440	180,380
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 805
Cumulative Number of Reported Claims | claim	52,003,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 184,489	177,834	166,815	140,901	73,073
Automobile liability | 2017
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	189,095	188,625	188,756	187,983
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 3,321
Cumulative Number of Reported Claims | claim	48,876,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 179,782	166,614	139,531	70,682
Automobile liability | 2018
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	192,866	195,284	200,314
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 12,931
Cumulative Number of Reported Claims | claim	47,684,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 168,628	141,537	77,528
Automobile liability | 2019
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	180,060	181,141
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 29,004
Cumulative Number of Reported Claims | claim	45,915,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 129,101	69,665
Automobile liability | 2020
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	136,977
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 51,452
Cumulative Number of Reported Claims | claim	30,032,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 51,486
Automobile physical damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	975,667
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	972,966
Outstanding prior to 2011	10
Prior years paid
Liabilities for claims and claim adjustment expenses, net of reinsurance	2,711
Automobile physical damage | 2011
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	85,089	85,090	85,102	85,108	85,116	85,143	85,120	86,023	85,507	86,205
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	80,804,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 85,082	85,090	85,095	85,108	85,116	85,127	85,148	85,181	85,254	$ 83,227
Automobile physical damage | 2012
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	83,323	83,322	83,334	83,342	83,354	83,431	83,402	82,337	83,770
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	78,166,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 83,318	83,322	83,326	83,342	83,347	83,355	83,372	83,418	$ 80,519
Automobile physical damage | 2013
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	88,452	88,457	88,525	88,455	88,627	88,672	88,856	91,448
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	80,920,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 88,442	88,452	88,471	88,484	88,532	88,580	88,688	$ 85,110
Automobile physical damage | 2014
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	95,242	95,241	95,232	95,239	95,422	95,634	95,572
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	87,901,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 95,228	95,243	95,258	95,256	95,266	95,444	$ 88,939
Automobile physical damage | 2015
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	97,608	97,612	97,455	97,624	97,994	99,291
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Reported Claims | claim	87,502,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 97,608	97,625	97,638	97,685	97,850	$ 92,138
Automobile physical damage | 2016
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	109,597	109,603	109,348	109,515	112,430
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ (18)
Cumulative Number of Reported Claims | claim	93,229,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 109,589	109,611	109,536	109,686	$ 106,459
Automobile physical damage | 2017
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	110,569	110,520	111,798	115,483
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ (17)
Cumulative Number of Reported Claims | claim	91,290,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 110,630	110,674	110,817	$ 105,156
Automobile physical damage | 2018
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	108,333	108,886	109,040
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ (161)
Cumulative Number of Reported Claims | claim	94,458,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 108,272	109,103	$ 103,559
Automobile physical damage | 2019
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	110,495	111,577
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ (326)
Cumulative Number of Reported Claims | claim	92,068,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 110,692	$ 106,243
Automobile physical damage | 2020
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	86,959
Total of Incurred- But-Not-Reported Liabilities Plus Expected Development on Reported Claims	$ (6,829)
Cumulative Number of Reported Claims | claim	66,650,000
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net of Reinsurance	$ 84,105