Additional Financial Data by Segment (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
Segment Reporting Information [Line Items]
Gross written premiums	$ 323,509	$ 354,062	$ 379,471	$ 396,460	$ 333,299	$ 327,469	$ 348,795	$ 422,790	$ 1,453,502	$ 1,432,353	$ 1,370,517
Ceded written premiums									(409,642)	(432,215)	(482,595)
Net written premiums	244,719	251,939	258,244	288,958	228,007	228,417	231,864	311,850	1,043,860	1,000,138	887,922
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
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums									865,781	807,108	745,995
Ceded written premiums									(268,775)	(254,983)	(279,769)
Net written premiums									597,006	552,125	466,226
Net earned premiums									555,836	504,289	437,737
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									158,124	154,233	143,867
Ceded written premiums									(61,916)	(46,685)	(44,359)
Net written premiums									96,208	107,548	99,508
Net earned premiums									96,082	105,650	106,150
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									596,673	543,045	472,049
Ceded written premiums									(152,168)	(169,953)	(202,430)
Net written premiums									444,505	373,092	269,619
Net earned premiums									401,408	314,833	231,186
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									110,984	109,830	130,079
Ceded written premiums									(54,691)	(38,345)	(32,980)
Net written premiums									56,293	71,485	97,099
Net earned premiums									58,346	83,806	100,401
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums									411,947	424,904	431,488
Ceded written premiums									(134,005)	(171,921)	(197,832)
Net written premiums									277,942	252,983	233,656
Net earned premiums									259,960	243,485	232,962
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									183,707	190,787	192,870
Ceded written premiums									(36,515)	(47,805)	(50,887)
Net written premiums									147,192	142,982	141,983
Net earned premiums									149,256	141,097	150,784
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									130,729	158,139	180,657
Ceded written premiums									(67,722)	(98,087)	(120,068)
Net written premiums									63,007	60,052	60,589
Net earned premiums									55,320	62,520	55,809
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									97,511	75,978	57,961
Ceded written premiums									(29,768)	(26,029)	(26,877)
Net written premiums									67,743	49,949	31,084
Net earned premiums									55,384	39,868	26,369
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									175,774	200,341	193,034
Ceded written premiums									(6,862)	(5,311)	(4,994)
Net written premiums									168,912	195,030	188,040
Net earned premiums									$ 168,291	$ 188,121	$ 171,240