Reinsurance Written Premium (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
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,330,265	$ 1,223,840	$ 1,183,120
Ceded to other companies									401,898	414,906	440,656
Assumed from other companies									171,978	175,153	193,431
Net earned premiums	$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 251,545	$ 254,082	$ 242,328	$ 236,132	1,100,345	984,087	935,895
P&C
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									1,403,865	1,277,728	1,232,012
Ceded to other companies									382,687	409,642	432,215
Assumed from other companies									165,046	175,774	200,341
Net earned premiums									$ 1,186,224	$ 1,043,860	$ 1,000,138
Percentage of amount assumed to net									14.00%	17.00%	20.00%