Additional Financial Data by Segment (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
Segment Reporting Information [Line Items]
Gross written premiums	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 323,509	$ 354,062	$ 379,471	$ 396,460	$ 1,568,911	$ 1,453,502	$ 1,432,353
Ceded written premiums									(382,687)	(409,642)	(432,215)
Net written premiums	282,868	277,001	306,535	319,820	244,719	251,939	258,244	288,958	1,186,224	1,043,860	1,000,138
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
Property Casualty
Segment Reporting Information [Line Items]
Net earned premiums									1,186,224	1,043,860	1,000,138
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums									919,395	865,781	807,108
Ceded written premiums									(235,827)	(268,775)	(254,983)
Net written premiums									683,568	597,006	552,125
Net earned premiums									629,308	555,836	504,289
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									169,405	158,124	154,233
Ceded written premiums									(70,858)	(61,916)	(46,685)
Net written premiums									98,547	96,208	107,548
Net earned premiums									100,132	96,082	105,650
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									631,562	596,673	543,045
Ceded written premiums									(135,888)	(152,168)	(169,953)
Net written premiums									495,674	444,505	373,092
Net earned premiums									453,673	401,408	314,833
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									118,428	110,984	109,830
Ceded written premiums									(29,081)	(54,691)	(38,345)
Net written premiums									89,347	56,293	71,485
Net earned premiums									75,503	58,346	83,806
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums									484,471	411,947	424,904
Ceded written premiums									(138,504)	(134,005)	(171,921)
Net written premiums									345,967	277,942	252,983
Net earned premiums									307,416	259,960	243,485
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									183,228	183,707	190,787
Ceded written premiums									(40,092)	(36,515)	(47,805)
Net written premiums									143,136	147,192	142,982
Net earned premiums									141,593	149,256	141,097
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									181,094	130,729	158,139
Ceded written premiums									(69,606)	(67,722)	(98,087)
Net written premiums									111,488	63,007	60,052
Net earned premiums									89,455	55,320	62,520
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									120,149	97,511	75,978
Ceded written premiums									(28,806)	(29,768)	(26,029)
Net written premiums									91,343	67,743	49,949
Net earned premiums									76,368	55,384	39,868
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									165,045	175,774	200,341
Ceded written premiums									(8,356)	(6,862)	(5,311)
Net written premiums									156,689	168,912	195,030
Net earned premiums									$ 163,621	$ 168,291	$ 188,121