Loss and Loss Adjustment Expenses - Development Tables of Incurred and Paid Claims and Allocated Claim Adjustment Expenses Net of Reinsurance (Detail) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($) Claim	Dec. 31, 2017 USD ($)		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]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 2,142,468
Total IBNR Plus Expected Development on Reported Claims		113,526
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		1,525,214
All outstanding claim liabilities before 2008, net of reinsurance		73,939
Liabilities for claims and claim adjustment expenses, net of reinsurance		691,193
Accident Year 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	174,731	$ 177,015		$ 176,586		$ 177,795		$ 182,274		$ 184,006		$ 187,943		$ 188,462		$ 188,746		$ 185,359
Total IBNR Plus Expected Development on Reported Claims		$ 3,407
Cumulative Number of Claims Reported | Claim		5,372
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 162,253	160,580	[1]	159,345	[1]	158,214	[1]	155,365	[1]	150,925	[1]	142,667	[1]	123,946	[1]	96,173	[1]	$ 42,332	[1]
Claim Frequency	[2]	19.82%
Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 189,040	189,403		191,000		193,029		198,861		205,769		208,035		202,479		179,156
Total IBNR Plus Expected Development on Reported Claims		$ 5,612
Cumulative Number of Claims Reported | Claim		5,970
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 176,663	175,019	[1]	172,426	[1]	169,565	[1]	165,834	[1]	156,974	[1]	141,029	[1]	108,714	[1]	$ 47,520	[1]
Claim Frequency	[2]	24.93%
Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 189,327	191,126		192,988		195,262		198,213		199,163		199,522		196,384
Total IBNR Plus Expected Development on Reported Claims		$ 7,045
Cumulative Number of Claims Reported | Claim		6,043
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 169,994	167,757	[1]	165,967	[1]	161,639	[1]	153,968	[1]	140,831	[1]	111,029	[1]	$ 53,329	[1]
Claim Frequency	[2]	22.81%
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 180,387	182,859		184,460		193,515		212,738		222,075		222,549
Total IBNR Plus Expected Development on Reported Claims		$ 7,454
Cumulative Number of Claims Reported | Claim		5,749
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 159,807	157,207	[1]	154,553	[1]	149,161	[1]	133,658	[1]	107,467	[1]	$ 50,579	[1]
Claim Frequency	[2]	18.73%
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 210,588	214,701		220,457		233,656		241,811		241,810
Total IBNR Plus Expected Development on Reported Claims		$ 10,210
Cumulative Number of Claims Reported | Claim		5,763
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 177,724	172,479	[1]	165,994	[1]	150,304	[1]	119,507	[1]	$ 51,396	[1]
Claim Frequency	[2]	16.54%
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 226,933	235,058		249,097		268,846		268,846
Total IBNR Plus Expected Development on Reported Claims		$ 12,576
Cumulative Number of Claims Reported | Claim		5,833
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 180,683	169,736	[1]	153,320	[1]	119,820	[1]	$ 53,060	[1]
Claim Frequency	[2]	14.97%
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 235,471	252,514		262,573		262,573
Total IBNR Plus Expected Development on Reported Claims		$ 22,038
Cumulative Number of Claims Reported | Claim		5,510
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 170,461	151,818	[1]	121,599	[1]	$ 54,141	[1]
Claim Frequency	[2]	14.23%
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 241,406	250,491		250,491
Total IBNR Plus Expected Development on Reported Claims		$ 19,874
Cumulative Number of Claims Reported | Claim		5,378
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 143,016	115,713	[1]	$ 52,238	[1]
Claim Frequency	[2]	14.19%
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 244,098	244,094
Total IBNR Plus Expected Development on Reported Claims		$ 9,887
Cumulative Number of Claims Reported | Claim		5,178
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 122,552	$ 56,951	[1]
Claim Frequency	[2]	14.58%
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	[1]	$ 250,487
Total IBNR Plus Expected Development on Reported Claims		$ 15,423
Cumulative Number of Claims Reported | Claim		5,186
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 62,061
Claim Frequency	[2]	14.42%

[1] Data presented for these calendar years is required supplementary information, which is unaudited

[2] Frequency, as calculated above, refers to reported claims divided by gross premium earned