Summary of Components of Net Losses and Loss Adjustment Expenses Incurred (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Direct									$ 1,002,358	$ 938,640	$ 795,414
Assumed									159,989	148,606	87,995
Ceded									(268,568)	(280,981)	(217,961)
Net Losses and LAE	$ 270,401	$ 240,900	$ 196,333	$ 186,145	$ 183,384	$ 276,171	$ 177,110	$ 169,600	$ 893,779	$ 806,265	$ 665,448