Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums	304,869	296,016	333,282	337,163	282,868	277,001	306,535	319,820	1,271,330	1,186,224	1,043,860
Net earned premiums	$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 283,985	$ 284,010	$ 267,992	$ 264,358	1,186,420	1,100,345	984,087
Property Casualty
Segment Reporting Information [Line Items]
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums									1,190,725	1,132,410	956,772
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums									988,293	919,395	865,781
Ceded written premiums									(266,993)	(235,827)	(268,775)
Net written premiums									721,300	683,568	597,006
Net earned premiums									674,665	629,308	555,836
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									156,171	169,405	158,124
Ceded written premiums									(72,431)	(70,858)	(61,916)
Net written premiums									83,740	98,547	96,208
Net earned premiums									86,605	100,132	96,082
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									713,539	631,562	596,673
Ceded written premiums									(173,501)	(135,888)	(152,168)
Net written premiums									540,038	495,674	444,505
Net earned premiums									495,260	453,673	401,408
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									118,583	118,428	110,984
Ceded written premiums									(21,061)	(29,081)	(54,691)
Net written premiums									97,522	89,347	56,293
Net earned premiums									92,800	75,503	58,346
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums									501,130	484,471	411,947
Ceded written premiums									(165,278)	(138,504)	(134,005)
Net written premiums									335,852	345,967	277,942
Net earned premiums									333,792	307,416	259,960
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									198,241	183,228	183,707
Ceded written premiums									(42,018)	(40,092)	(36,515)
Net written premiums									156,223	143,136	147,192
Net earned premiums									152,396	141,593	149,256
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									159,123	181,094	130,729
Ceded written premiums									(85,298)	(69,606)	(67,722)
Net written premiums									73,825	111,488	63,007
Net earned premiums									88,517	89,455	55,320
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									143,766	120,149	97,511
Ceded written premiums									(37,962)	(28,806)	(29,768)
Net written premiums									105,804	91,343	67,743
Net earned premiums									92,879	76,368	55,384
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									223,842	165,045	175,774
Ceded written premiums									(9,664)	(8,356)	(6,862)
Net written premiums									214,178	156,689	168,912
Net earned premiums									$ 177,963	$ 163,621	$ 168,291