Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 464,558	$ 455,943	$ 497,236	$ 495,224	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 1,912,961	$ 1,713,265	$ 1,568,911
Ceded written premiums									(441,876)	(441,935)	(382,687)
Net written premiums	346,119	352,412	379,292	393,262	304,869	296,016	333,282	337,163	1,471,085	1,271,330	1,186,224
Net earned premiums	$ 362,543	$ 347,038	$ 331,015	$ 322,627	$ 305,099	$ 301,355	$ 293,835	$ 286,131	1,363,223	1,186,420	1,100,345
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums									1,083,116	988,293	919,395
Ceded written premiums									(283,025)	(266,993)	(235,827)
Net written premiums									800,091	721,300	683,568
Net earned premiums									737,646	674,665	629,308
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									130,872	118,583	118,428
Ceded written premiums									(19,621)	(21,061)	(29,081)
Net written premiums									111,251	97,522	89,347
Net earned premiums									103,736	92,800	75,503
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									148,485	156,171	169,405
Ceded written premiums									(64,097)	(72,431)	(70,858)
Net written premiums									84,388	83,740	98,547
Net earned premiums									84,774	86,605	100,132
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									803,759	713,539	631,562
Ceded written premiums									(199,307)	(173,501)	(135,888)
Net written premiums									604,452	540,038	495,674
Net earned premiums									549,136	495,260	453,673
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums									530,710	501,130	484,471
Ceded written premiums									(147,402)	(165,278)	(138,504)
Net written premiums									383,308	335,852	345,967
Net earned premiums									380,503	333,792	307,416
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									178,155	143,766	120,149
Ceded written premiums									(38,870)	(37,962)	(28,806)
Net written premiums									139,285	105,804	91,343
Net earned premiums									124,063	92,879	76,368
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									181,053	198,241	183,228
Ceded written premiums									(44,356)	(42,018)	(40,092)
Net written premiums									136,697	156,223	143,136
Net earned premiums									151,296	152,396	141,593
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									171,502	159,123	181,094
Ceded written premiums									(64,176)	(85,298)	(69,606)
Net written premiums									107,326	73,825	111,488
Net earned premiums									105,144	88,517	89,455
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									299,135	223,842	165,045
Ceded written premiums									(11,449)	(9,664)	(8,356)
Net written premiums									287,686	214,178	156,689
Net earned premiums									$ 245,074	$ 177,963	$ 163,621