Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 452,179	$ 412,565	$ 902,484	$ 826,442
Ceded Written Premiums	(118,897)	(106,030)	(232,039)	(200,087)
Net Written Premiums	333,282	306,535	670,445	626,355
Net Earned Premiums	$ 293,835	267,992	$ 579,966	532,350
% Change Gross Written Premiums	9.60%		9.20%
% Change Ceded Written Premiums	12.10%		16.00%
% Change Net Written Premiums	8.70%		7.00%
% Change Net Earned Premiums	9.60%		8.90%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 257,186	243,924	$ 494,791	454,571
Ceded Written Premiums	(73,857)	(62,618)	(135,344)	(117,115)
Net Written Premiums	183,329	181,306	359,447	337,456
Net Earned Premiums	$ 167,087	152,384	$ 331,091	300,724
% Change Gross Written Premiums	5.40%		8.80%
% Change Ceded Written Premiums	17.90%		15.60%
% Change Net Written Premiums	1.10%		6.50%
% Change Net Earned Premiums	9.60%		10.10%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 41,687	46,187	$ 82,637	89,350
Ceded Written Premiums	(19,451)	(18,540)	(36,971)	(36,187)
Net Written Premiums	22,236	27,647	45,666	53,163
Net Earned Premiums	$ 21,812	24,921	$ 44,506	48,171
% Change Gross Written Premiums	(9.70%)		(7.50%)
% Change Ceded Written Premiums	4.90%		2.20%
% Change Net Written Premiums	(19.60%)		(14.10%)
% Change Net Earned Premiums	(12.50%)		(7.60%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 187,492	168,654	$ 358,126	309,932
Ceded Written Premiums	(51,147)	(36,768)	(89,345)	(67,275)
Net Written Premiums	136,345	131,886	268,781	242,657
Net Earned Premiums	$ 121,226	110,212	$ 240,349	219,371
% Change Gross Written Premiums	11.20%		15.50%
% Change Ceded Written Premiums	39.10%		32.80%
% Change Net Written Premiums	3.40%		10.80%
% Change Net Earned Premiums	10.00%		9.60%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 28,007	29,083	$ 54,028	55,289
Ceded Written Premiums	(3,259)	(7,310)	(9,028)	(13,653)
Net Written Premiums	24,748	21,773	45,000	41,636
Net Earned Premiums	$ 24,049	17,251	$ 46,236	33,182
% Change Gross Written Premiums	(3.70%)		(2.30%)
% Change Ceded Written Premiums	(55.40%)		(33.90%)
% Change Net Written Premiums	13.70%		8.10%
% Change Net Earned Premiums	39.40%		39.30%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 131,193	135,740	$ 273,053	279,883
Ceded Written Premiums	(43,809)	(41,910)	(90,402)	(76,881)
Net Written Premiums	87,384	93,830	182,651	203,002
Net Earned Premiums	$ 82,100	77,833	$ 166,186	155,841
% Change Gross Written Premiums	(3.30%)		(2.40%)
% Change Ceded Written Premiums	4.50%		17.60%
% Change Net Written Premiums	(6.90%)		(10.00%)
% Change Net Earned Premiums	5.50%		6.60%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 49,597	41,147	$ 118,430	113,095
Ceded Written Premiums	(12,536)	(9,554)	(23,462)	(20,643)
Net Written Premiums	37,061	31,593	94,968	92,452
Net Earned Premiums	$ 39,525	36,096	$ 77,020	74,952
% Change Gross Written Premiums	20.50%		4.70%
% Change Ceded Written Premiums	31.20%		13.70%
% Change Net Written Premiums	17.30%		2.70%
% Change Net Earned Premiums	9.50%		2.80%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 46,663	63,002	$ 87,031	107,048
Ceded Written Premiums	(20,362)	(24,465)	(50,008)	(41,694)
Net Written Premiums	26,301	38,537	37,023	65,354
Net Earned Premiums	$ 23,337	23,815	$ 45,517	45,024
% Change Gross Written Premiums	(25.90%)		(18.70%)
% Change Ceded Written Premiums	(16.80%)		19.90%
% Change Net Written Premiums	(31.80%)		(43.40%)
% Change Net Earned Premiums	(2.00%)		1.10%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 34,933	31,591	$ 67,592	59,740
Ceded Written Premiums	(10,911)	(7,891)	(16,932)	(14,544)
Net Written Premiums	24,022	23,700	50,660	45,196
Net Earned Premiums	$ 19,238	17,922	$ 43,649	35,865
% Change Gross Written Premiums	10.60%		13.10%
% Change Ceded Written Premiums	38.30%		16.40%
% Change Net Written Premiums	1.40%		12.10%
% Change Net Earned Premiums	7.30%		21.70%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 63,800	32,901	$ 134,640	91,988
Ceded Written Premiums	(1,231)	(1,502)	(6,293)	(6,091)
Net Written Premiums	62,569	31,399	128,347	85,897
Net Earned Premiums	$ 44,648	$ 37,775	$ 82,689	$ 75,785
% Change Gross Written Premiums	93.90%		46.40%
% Change Ceded Written Premiums	(18.10%)		3.30%
% Change Net Written Premiums	99.30%		49.40%
% Change Net Earned Premiums	18.20%		9.10%