Subsequent Events - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Oct. 10, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Subsequent Event [Line Items]
Direct Losses and Loss Adjustment Expenses		$ 270,158,000	$ 531,268,000	$ 593,419,000	$ 687,707,000
Losses and loss adjustment expenses		$ 85,947,000	$ 116,375,000	$ 251,715,000	$ 267,129,000
Hurricane Michael [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Losses and loss adjustment expenses	$ 35,000,000
Hurricane Michael [Member] | Minimum [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Direct Losses and Loss Adjustment Expenses	300,000,000
Hurricane Michael [Member] | Maximum [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Direct Losses and Loss Adjustment Expenses	$ 350,000,000