Summary of Components of Net Earned Premium (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance [Abstract]
Direct									$ 1,223,840	$ 1,183,120	$ 1,121,198
Assumed									175,153	193,431	176,726
Ceded									(414,906)	(440,656)	(455,985)
Net earned premiums	$ 251,545	$ 254,082	$ 242,328	$ 236,132	$ 234,172	$ 245,367	$ 231,084	$ 225,272	$ 984,087	$ 935,895	$ 841,939