Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct business									$ 468,046		$ 458,185		$ 229,978
Reinsurance assumed									97,799		132,048		61,275
Reinsurance ceded									(94,905)	[1]	(88,989)	[1]	(18,072)
Net premiums									470,940		501,244		273,181
Direct business									466,750		452,441		228,652
Reinsurance assumed									98,267		144,554		58,414
Reinsurance ceded									(96,552)	[1]	(92,852)	[1]	(18,547)
Net premiums	$ 109,472	$ 119,553	$ 117,804	$ 121,636	$ 123,222	$ 124,707	$ 128,877	$ 127,337	$ 468,465		$ 504,143		$ 268,519

[1] Includes ceded written premiums and ceded earned premiums of $55.8 million and $59.5 million, respectively, to American Bankers Insurance Company.