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, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	[1]	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]	Dec. 31, 2011 USD ($)	[1]	Dec. 31, 2010 USD ($)	[1]	Dec. 31, 2009 USD ($)	[1]	Dec. 31, 2008 USD ($)	[1]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 2,119,907
Total IBNR Plus Expected Development on Reported Claims		98,504
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		1,500,397
All outstanding claim liabilities before 2008, net of reinsurance		67,446
Liabilities for claims and claim adjustment expenses, net of reinsurance		686,956
Accident Year 2008 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		182,646	$ 182,901		$ 183,608		$ 182,846		$ 184,547		$ 187,695		$ 190,905		$ 195,619		$ 200,371		$ 196,923
Total IBNR Plus Expected Development on Reported Claims		$ 2,649
Cumulative Number of Claims Reported | Claim		6,487
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 173,138	171,496		170,347		166,814		163,933		157,388		147,377		132,515		103,057		$ 47,685
Claim Frequency	[2]	20.98%
Accident Year 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 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		$ 3,345
Cumulative Number of Claims Reported | Claim		5,371
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 160,580	159,345		158,214		155,365		150,925		142,667		123,946		96,173		$ 42,332
Claim Frequency	[2]	19.82%
Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 189,403	191,000		193,029		198,861		205,769		208,035		202,479		179,156
Total IBNR Plus Expected Development on Reported Claims		$ 5,805
Cumulative Number of Claims Reported | Claim		5,971
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 175,019	172,426		169,565		165,834		156,974		141,029		108,714		$ 47,520
Claim Frequency	[2]	24.94%
Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 191,126	192,988		195,262		198,213		199,163		199,522		196,384
Total IBNR Plus Expected Development on Reported Claims		$ 6,968
Cumulative Number of Claims Reported | Claim		6,041
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 167,757	165,967		161,639		153,968		140,831		111,029		$ 53,329
Claim Frequency	[2]	22.81%
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 182,859	184,460		193,515		212,738		222,075		222,549
Total IBNR Plus Expected Development on Reported Claims		$ 7,840
Cumulative Number of Claims Reported | Claim		5,746
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 157,206	154,553		149,161		133,658		107,467		$ 50,579
Claim Frequency	[2]	18.72%
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 214,701	220,457		233,656		241,811		241,810
Total IBNR Plus Expected Development on Reported Claims		$ 10,501
Cumulative Number of Claims Reported | Claim		5,762
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 172,479	165,994		150,304		119,507		$ 51,396
Claim Frequency	[2]	16.54%
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 235,058	249,097		268,846		268,846
Total IBNR Plus Expected Development on Reported Claims		$ 13,503
Cumulative Number of Claims Reported | Claim		5,827
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 169,736	153,320		119,820		$ 53,060
Claim Frequency	[2]	14.96%
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 252,514	262,573		262,573
Total IBNR Plus Expected Development on Reported Claims		$ 22,863
Cumulative Number of Claims Reported | Claim		5,506
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 151,818	121,599		$ 54,141
Claim Frequency	[2]	14.22%
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 250,491	250,491
Total IBNR Plus Expected Development on Reported Claims		$ 17,066
Cumulative Number of Claims Reported | Claim		5,346
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 115,713	$ 52,238
Claim Frequency	[2]	14.11%
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 244,094
Total IBNR Plus Expected Development on Reported Claims		$ 7,964
Cumulative Number of Claims Reported | Claim		4,919
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 56,951
Claim Frequency	[2]	13.86%

[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.