Loss and Loss Adjustment Expenses - Development Tables of Incurred and Paid Losses and Allocated Loss Adjustment Expenses Net of Reinsurance (Detail) $ in Thousands		12 Months Ended
		Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	[1]	Dec. 31, 2017 USD ($)	[1]	Dec. 31, 2016 USD ($)	[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]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 2,147,866
Total IBNR Plus Expected Development on Reported Claims		104,646
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		1,551,154
All outstanding liabilities before 2010, net of reinsurance		80,832
Liabilities for loss and loss adjustment expenses, net of reinsurance		677,544
Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		188,096	$ 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		$ 5,093
Cumulative Number of Claims Reported | Claim		5,971
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 177,331	176,663		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 Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 188,226	189,327		191,126		192,988		195,262		198,213		199,163		199,522		196,384
Total IBNR Plus Expected Development on Reported Claims		$ 6,008
Cumulative Number of Claims Reported | Claim		6,044
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 170,785	169,994		167,757		165,967		161,639		153,968		140,831		111,029		$ 53,329
Claim Frequency	[2]	22.82%
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 178,586	180,387		182,859		184,460		193,515		212,738		222,075		222,549
Total IBNR Plus Expected Development on Reported Claims		$ 6,834
Cumulative Number of Claims Reported | Claim		5,749
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 161,014	159,807		157,207		154,553		149,161		133,658		107,467		$ 50,579
Claim Frequency	[2]	18.73%
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 209,184	210,588		214,701		220,457		233,656		241,811		241,810
Total IBNR Plus Expected Development on Reported Claims		$ 8,803
Cumulative Number of Claims Reported | Claim		5,766
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 180,614	177,724		172,479		165,994		150,304		119,507		$ 51,396
Claim Frequency	[2]	16.55%
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 218,386	226,933		235,058		249,097		268,846		268,846
Total IBNR Plus Expected Development on Reported Claims		$ 6,534
Cumulative Number of Claims Reported | Claim		5,838
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 186,129	180,683		169,736		153,320		119,820		$ 53,060
Claim Frequency	[2]	14.99%
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 220,965	235,471		252,514		262,573		262,573
Total IBNR Plus Expected Development on Reported Claims		$ 12,488
Cumulative Number of Claims Reported | Claim		5,515
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 182,053	170,461		151,818		121,599		$ 54,141
Claim Frequency	[2]	14.25%
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 218,005	241,406		250,491		250,491
Total IBNR Plus Expected Development on Reported Claims		$ 11,377
Cumulative Number of Claims Reported | Claim		5,390
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 156,861	143,016		115,713		$ 52,238
Claim Frequency	[2]	14.22%
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 234,587	244,098		244,094
Total IBNR Plus Expected Development on Reported Claims		$ 14,636
Cumulative Number of Claims Reported | Claim		5,199
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 151,427	122,552		$ 56,951
Claim Frequency	[2]	14.64%
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 250,487	250,487
Total IBNR Plus Expected Development on Reported Claims		$ 13,900
Cumulative Number of Claims Reported | Claim		5,451
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 126,057	$ 62,061
Claim Frequency	[2]	15.16%
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 241,344
Total IBNR Plus Expected Development on Reported Claims		$ 18,973
Cumulative Number of Claims Reported | Claim		4,959
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 58,883
Claim Frequency	[2]	14.50%

[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