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 ($)	Dec. 31, 2021 USD ($) Claim		Dec. 31, 2019 USD ($)	[1]	Dec. 31, 2018 USD ($)	[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]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance			$ 2,103,828
Total IBNR Plus Expected Development on Reported Claims			75,594
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]		1,564,869
All outstanding liabilities before 2012, net of reinsurance			87,052
Liabilities for loss and loss adjustment expenses, net of reinsurance			626,012
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 177,547	176,843		$ 178,586		$ 180,387		$ 182,859		$ 184,460		$ 193,515		$ 212,738		$ 222,075		$ 222,549
Total IBNR Plus Expected Development on Reported Claims			$ 5,481
Cumulative Number of Claims Reported | Claim			5,749
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 162,588	$ 164,337	[1]	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	205,421		209,184		210,588		214,701		220,457		233,656		241,811		241,810
Total IBNR Plus Expected Development on Reported Claims			$ 7,031
Cumulative Number of Claims Reported | Claim			5,768
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 182,465	$ 183,365	[1]	180,614		177,724		172,479		165,994		150,304		119,507		$ 51,396
Claim Frequency	[2]	16.56%
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 212,417	210,811		218,386		226,933		235,058		249,097		268,846		268,846
Total IBNR Plus Expected Development on Reported Claims			$ 4,508
Cumulative Number of Claims Reported | Claim			5,840
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 191,394	$ 192,168	[1]	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	207,929		220,965		235,471		252,514		262,573		262,573
Total IBNR Plus Expected Development on Reported Claims			$ 4,397
Cumulative Number of Claims Reported | Claim			5,516
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 185,657	$ 188,145	[1]	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	202,820		218,005		241,406		250,491		250,491
Total IBNR Plus Expected Development on Reported Claims			$ 5,674
Cumulative Number of Claims Reported | Claim			5,393
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 166,887	$ 172,133	[1]	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	211,964		234,587		244,098		244,094
Total IBNR Plus Expected Development on Reported Claims			$ 4,910
Cumulative Number of Claims Reported | Claim			5,212
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 166,448	$ 175,733	[1]	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	217,369		250,487		250,487
Total IBNR Plus Expected Development on Reported Claims			$ 8,241
Cumulative Number of Claims Reported | Claim			5,471
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 152,328	$ 172,423	[1]	126,057		$ 62,061
Claim Frequency	[2]	15.21%
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 241,344	227,246		241,344
Total IBNR Plus Expected Development on Reported Claims			$ 10,308
Cumulative Number of Claims Reported | Claim			5,213
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 120,512	$ 154,391	[1]	$ 58,883
Claim Frequency	[2]	15.25%
Accident Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 220,710	220,710
Total IBNR Plus Expected Development on Reported Claims			$ 10,919
Cumulative Number of Claims Reported | Claim			4,376
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance		$ 50,113	$ 109,882	[1]
Claim Frequency	[2]	13.91%
Accident Year 2021 [Member]
Claims Development [Line Items]
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance			222,715
Total IBNR Plus Expected Development on Reported Claims			$ 14,125
Cumulative Number of Claims Reported | Claim			4,080
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	[1]		$ 52,292
Claim Frequency	[2]	14.24%

[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