Reserve for Unpaid Losses - Summary of Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,385,969
Net IBNR Reserves	161,137
Accident Year 2012 And Prior [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	106,628	$ 106,446	$ 106,654	$ 106,331	$ 106,493	$ 107,842	$ 105,765	$ 102,723
Net IBNR Reserves	$ 74
Reported Claims | Claim	53,353
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 62,564	62,378	62,354	62,166	62,969	61,483	$ 61,157
Net IBNR Reserves	$ 359
Reported Claims | Claim	13,095
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 115,234	115,838	114,984	113,847	114,899	$ 118,991
Net IBNR Reserves	$ 975
Reported Claims | Claim	18,477
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 206,011	205,164	203,792	197,744	$ 179,255
Net IBNR Reserves	$ 4,106
Reported Claims | Claim	26,054
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 250,235	250,990	242,611	$ 237,207
Net IBNR Reserves	$ 7,105
Reported Claims | Claim	27,495
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 192,749	195,240	$ 189,163
Net IBNR Reserves	$ 8,374
Reported Claims | Claim	69,967
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 193,672	$ 199,565
Net IBNR Reserves	$ 30,125
Reported Claims | Claim	33,482
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 258,876
Net IBNR Reserves	$ 110,019
Reported Claims | Claim	23,098