Reserve for Unpaid Losses - Summary of Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 959,776
Accident Year 2012 And Prior [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	105,875	$ 105,053	$ 104,362	$ 104,093	$ 102,750	$ 101,323	$ 92,909
Accident Year 2013 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	60,581	59,395	57,647	55,589	50,716	$ 35,771
Accident Year 2014 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	112,518	108,509	101,456	95,076	$ 68,732
Accident Year 2015 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	192,967	181,672	162,654	$ 103,918
Accident Year 2016 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	233,540	211,512	$ 132,679
Accident Year 2017 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	169,743	$ 103,148
Accident Year 2018 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 84,552