Loss And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands	Dec. 31, 2018 USD ($) number_of_claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Claims Development [Line Items]
Net outstanding liabilities	$ 1,555,579
Property insurance product line, automobile [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	15,423,477
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	14,128,949
All outstanding liabilities before 2009, net of reinsurance	(505)
Net outstanding liabilities	1,294,025
Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,153,646
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	2,002,020
All outstanding liabilities before 2009, net of reinsurance	2,416
Net outstanding liabilities	154,043
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	1,371,971	$ 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	$ 109
Cumulative Number of Reported Claims | number_of_claims	186
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,372,090	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 2007 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	134,374	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	$ 62
Cumulative Number of Reported Claims | number_of_claims	17
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 134,193	134,229	134,301	134,378	134,381	132,180	130,928	126,591	119,306	$ 86,034
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	1,388,055	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	$ 171
Cumulative Number of Reported Claims | number_of_claims	184
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2008 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ 136
Cumulative Number of Reported Claims | number_of_claims	21
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 159,435	159,268	158,943	158,053	156,853	155,191	149,084	137,628	$ 95,057
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	1,401,178	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	$ 443
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2009 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ 310
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 169,757	168,914	168,621	167,806	166,375	162,870	153,845	$ 111,909
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	1,438,250	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	$ 1,075
Cumulative Number of Reported Claims | number_of_claims	181
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 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 2010 [Member] | Property insurance product line, homeowners' [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	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	$ 674
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 191,660	190,649	190,373	188,121	182,756	175,029	$ 128,618
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	1,468,294	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	$ 2,818
Cumulative Number of Reported Claims | number_of_claims	185
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,460,352	1,447,004	1,413,999	1,340,724	1,217,906	974,445
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	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	$ 780
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 186,132	185,168	183,860	180,858	174,295	$ 133,528
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	1,498,504	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	$ 6,626
Cumulative Number of Reported Claims | number_of_claims	180
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,476,944	1,432,472	1,358,472	1,231,413	967,481
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	204,986	202,513	203,218	202,621	199,298
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 1,341
Cumulative Number of Reported Claims | number_of_claims	25
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 202,193	198,758	194,605	186,996	$ 139,615
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	1,634,435	1,610,839	1,588,443	1,551,105
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 19,491
Cumulative Number of Reported Claims | number_of_claims	170
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,560,480	1,466,368	1,336,223	1,040,253
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	236,855	233,501	234,881	234,800
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 3,131
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 230,480	224,178	213,994	$ 163,196
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	1,713,696	1,669,642	1,672,853
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 50,261
Cumulative Number of Reported Claims | number_of_claims	154
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,554,217	1,395,199	1,094,006
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	259,497	259,489	250,691
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 6,525
Cumulative Number of Reported Claims | number_of_claims	24
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 245,878	234,215	$ 173,537
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	1,727,277	1,703,857
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 133,587
Cumulative Number of Reported Claims | number_of_claims	148
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,399,202	1,076,079
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	295,163	309,491
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 12,186
Cumulative Number of Reported Claims | number_of_claims	30
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 269,254	$ 217,900
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	1,781,817
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 390,346
Cumulative Number of Reported Claims | number_of_claims	139
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,082,127
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	311,798
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims	$ 59,689
Cumulative Number of Reported Claims | number_of_claims	23
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 213,038