Reserve for Unpaid Losses - Summary of Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	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	$ 491,603
Net IBNR Reserves	82,213
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	797	$ 784	$ 811	$ 851	$ 1,396
Net IBNR Reserves	$ 6
Reported Claims | Claim	125
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 37,090	37,212	35,819	$ 37,005
Net IBNR Reserves	$ 531
Reported Claims | Claim	3,607
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 86,634	86,508	$ 91,839
Net IBNR Reserves	$ 2,155
Reported Claims | Claim	7,770
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 159,899	$ 141,125
Net IBNR Reserves	$ 13,637
Reported Claims | Claim	11,068
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 207,183
Net IBNR Reserves	$ 65,884
Reported Claims | Claim	14,603