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, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 2,089,136
Total IBNR plus Expected Development on Reported Claims		124,882
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		1,484,933
All outstanding claim liabilities before 2007, net of reinsurance		60,317
Liabilities for claims and claim adjustment expenses, net of reinsurance		664,520
Accident Year 2007 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		178,582	$ 178,608	$ 179,373	$ 180,229	$ 181,633	$ 185,225	$ 196,773	$ 206,044	$ 218,448	$ 208,480
Total IBNR plus Expected Development on Reported Claims		$ 1,620
Cumulative Number of Claims Reported | Claim		7,122
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 167,996	166,734	164,970	162,903	158,056	152,136	141,595	128,254	97,657	$ 43,470
Claim Frequency	[1]	21.77%
Accident Year 2008 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 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		$ 2,751
Cumulative Number of Claims Reported | Claim		6,486
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 171,496	170,347	166,814	163,933	157,388	147,377	132,515	103,057	$ 47,685
Claim Frequency	[1]	20.98%
Accident Year 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 176,586	177,795	182,274	184,006	187,943	188,462	188,746	185,359
Total IBNR plus Expected Development on Reported Claims		$ 3,199
Cumulative Number of Claims Reported | Claim		5,371
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 159,345	158,214	155,365	150,925	142,667	123,946	96,173	$ 42,332
Claim Frequency	[1]	19.82%
Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 191,000	193,029	198,861	205,769	208,035	202,479	179,156
Total IBNR plus Expected Development on Reported Claims		$ 5,786
Cumulative Number of Claims Reported | Claim		5,970
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 172,426	169,565	165,834	156,974	141,029	108,714	$ 47,520
Claim Frequency	[1]	24.93%
Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 192,988	195,262	198,213	199,163	199,522	196,384
Total IBNR plus Expected Development on Reported Claims		$ 7,461
Cumulative Number of Claims Reported | Claim		6,040
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 165,967	161,639	153,968	140,831	111,029	$ 53,329
Claim Frequency	[1]	22.80%
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 184,460	193,515	212,738	222,075	222,549
Total IBNR plus Expected Development on Reported Claims		$ 7,798
Cumulative Number of Claims Reported | Claim		5,743
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 154,553	149,161	133,658	107,467	$ 50,579
Claim Frequency	[1]	18.71%
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 220,457	233,656	241,811	241,810
Total IBNR plus Expected Development on Reported Claims		$ 11,868
Cumulative Number of Claims Reported | Claim		5,761
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 165,994	150,304	119,507	$ 51,396
Claim Frequency	[1]	16.54%
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 249,097	268,846	268,846
Total IBNR plus Expected Development on Reported Claims		$ 21,004
Cumulative Number of Claims Reported | Claim		5,815
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 153,320	119,820	$ 53,060
Claim Frequency	[1]	14.93%
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 262,573	262,573
Total IBNR plus Expected Development on Reported Claims		$ 28,658
Cumulative Number of Claims Reported | Claim		5,477
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 121,599	$ 54,141
Claim Frequency	[1]	14.15%
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 250,491
Total IBNR plus Expected Development on Reported Claims		$ 34,737
Cumulative Number of Claims Reported | Claim		5,104
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance		$ 52,238
Claim Frequency	[1]	13.47%

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