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, 2020 USD ($) Claim	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)		Dec. 31, 2013 USD ($)		Dec. 31, 2012 USD ($)		Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 2,124,295
Total IBNR Plus Expected Development on Reported Claims		87,630
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	1,549,975
All outstanding liabilities before 2011, net of reinsurance		80,533
Liabilities for loss and loss adjustment expenses, net of reinsurance		654,854
Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		188,264	$ 188,226	[1]	$ 189,327	[1]	$ 191,126	[1]	$ 192,988	[1]	$ 195,262	[1]	$ 198,213	[1]	$ 199,163	[1]	$ 199,522	[1]	$ 196,384	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 5,198
Cumulative Number of Claims Reported | Claim		6,044
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 171,583	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		$ 177,547	178,586	[1]	180,387	[1]	182,859	[1]	184,460	[1]	193,515	[1]	212,738	[1]	222,075	[1]	222,549	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 6,419
Cumulative Number of Claims Reported | Claim		5,749
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 162,588	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		$ 207,304	209,184	[1]	210,588	[1]	214,701	[1]	220,457	[1]	233,656	[1]	241,811	[1]	241,810	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 7,762
Cumulative Number of Claims Reported | Claim		5,766
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 182,465	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		$ 212,417	218,386	[1]	226,933	[1]	235,058	[1]	249,097	[1]	268,846	[1]	268,846	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 5,468
Cumulative Number of Claims Reported | Claim		5,839
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 191,394	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		$ 211,758	220,965	[1]	235,471	[1]	252,514	[1]	262,573	[1]	262,573	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 6,717
Cumulative Number of Claims Reported | Claim		5,516
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 185,657	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		$ 209,214	218,005	[1]	241,406	[1]	250,491	[1]	250,491	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 6,843
Cumulative Number of Claims Reported | Claim		5,393
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 166,887	156,861		143,016		115,713		$ 52,238
Claim Frequency	[2]	14.23%
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 220,096	234,587	[1]	244,098	[1]	244,094	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 8,066
Cumulative Number of Claims Reported | Claim		5,211
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 166,448	151,427		122,552		$ 56,951
Claim Frequency	[2]	14.68%
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 235,641	250,487	[1]	250,487	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 16,057
Cumulative Number of Claims Reported | Claim		5,467
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 152,328	126,057		$ 62,061
Claim Frequency	[2]	15.20%
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 241,344	241,344	[1]
Total IBNR Plus Expected Development on Reported Claims		$ 16,609
Cumulative Number of Claims Reported | Claim		5,199
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 120,512	$ 58,883
Claim Frequency	[2]	15.21%
Accident Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 220,710
Total IBNR Plus Expected Development on Reported Claims		$ 8,491
Cumulative Number of Claims Reported | Claim		4,170
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]	$ 50,113
Claim Frequency	[2]	13.25%

[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