Summary of Components of Net Losses and Loss Adjustment Expenses Incurred (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Direct									$ 938,640	$ 795,414	$ 703,361
Assumed									148,606	87,995	97,947
Ceded									(280,981)	(217,961)	(228,710)
Net Losses and LAE	$ 183,384	$ 276,171	$ 177,110	$ 169,600	$ 172,493	$ 172,793	$ 167,206	$ 152,956	$ 806,265	$ 665,448	$ 572,598