Reinsurance Written 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
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,223,840	$ 1,183,120	$ 1,121,198
Ceded to other companies									414,906	440,656	455,985
Assumed from other companies									175,153	193,431	176,726
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
Property Casualty
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									1,277,728	1,232,012	1,177,483
Ceded to other companies									409,642	432,215	482,595
Assumed from other companies									175,774	200,341	193,034
Net earned premiums									$ 1,043,860	$ 1,000,138	$ 887,922
Percentage of amount assumed to net									17.00%	20.00%	22.00%