Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 412,565	$ 379,471	$ 826,442	$ 775,931
Ceded written premiums	(106,030)	(121,227)	(200,087)	(228,729)
Net written premiums	306,535	258,244	626,355	547,202
Net earned premiums	$ 267,992	242,328	$ 532,350	478,460
% Change Gross written premiums	8.70%		6.50%
% Change Ceded written premiums	(12.50%)		(12.50%)
% Change Net written premiums	18.70%		14.50%
% Change Net earned premiums	10.60%		11.30%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 243,924	239,987	$ 454,571	439,314
Ceded written premiums	(62,618)	(78,882)	(117,115)	(147,612)
Net written premiums	181,306	161,105	337,456	291,702
Net earned premiums	$ 152,384	137,304	$ 300,724	268,395
% Change Gross written premiums	1.60%		3.50%
% Change Ceded written premiums	(20.60%)		(20.70%)
% Change Net written premiums	12.50%		15.70%
% Change Net earned premiums	11.00%		12.00%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 46,187	41,803	$ 89,350	82,638
Ceded written premiums	(18,540)	(15,026)	(36,187)	(31,312)
Net written premiums	27,647	26,777	53,163	51,326
Net earned premiums	$ 24,921	25,977	$ 48,171	49,901
% Change Gross written premiums	10.50%		8.10%
% Change Ceded written premiums	23.40%		15.60%
% Change Net written premiums	3.20%		3.60%
% Change Net earned premiums	(4.10%)		(3.50%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 168,654	170,216	$ 309,932	304,362
Ceded written premiums	(36,768)	(49,305)	(67,275)	(88,863)
Net written premiums	131,886	120,911	242,657	215,499
Net earned premiums	$ 110,212	96,440	$ 219,371	187,175
% Change Gross written premiums	(0.90%)		1.80%
% Change Ceded written premiums	(25.40%)		(24.30%)
% Change Net written premiums	9.10%		12.60%
% Change Net earned premiums	14.30%		17.20%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 29,083	27,968	$ 55,289	52,314
Ceded written premiums	(7,310)	(14,551)	(13,653)	(27,437)
Net written premiums	21,773	13,417	41,636	24,877
Net earned premiums	$ 17,251	14,887	$ 33,182	31,319
% Change Gross written premiums	4.00%		5.70%
% Change Ceded written premiums	(49.80%)		(50.20%)
% Change Net written premiums	62.30%		67.40%
% Change Net earned premiums	15.90%		5.90%
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 135,740	108,790	$ 279,883	231,217
Ceded written premiums	(41,910)	(41,621)	(76,881)	(75,877)
Net written premiums	93,830	67,169	203,002	155,340
Net earned premiums	$ 77,833	63,186	$ 155,841	128,711
% Change Gross written premiums	24.80%		21.00%
% Change Ceded written premiums	0.70%		1.30%
% Change Net written premiums	39.70%		30.70%
% Change Net earned premiums	23.20%		21.10%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 41,147	41,432	$ 113,095	108,842
Ceded written premiums	(9,554)	(9,126)	(20,643)	(19,948)
Net written premiums	31,593	32,306	92,452	88,894
Net earned premiums	$ 36,096	40,132	$ 74,952	77,924
% Change Gross written premiums	(0.70%)		3.90%
% Change Ceded written premiums	4.70%		3.50%
% Change Net written premiums	(2.20%)		4.00%
% Change Net earned premiums	(10.10%)		(3.80%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 63,002	41,615	$ 107,048	74,565
Ceded written premiums	(24,465)	(24,513)	(41,694)	(40,642)
Net written premiums	38,537	17,102	65,354	33,923
Net earned premiums	$ 23,815	10,199	$ 45,024	26,233
% Change Gross written premiums	51.40%		43.60%
% Change Ceded written premiums	(0.20%)		2.60%
% Change Net written premiums	125.30%		92.70%
% Change Net earned premiums	133.50%		71.60%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 31,591	25,743	$ 59,740	47,810
Ceded written premiums	(7,891)	(7,982)	(14,544)	(15,287)
Net written premiums	23,700	17,761	45,196	32,523
Net earned premiums	$ 17,922	12,855	$ 35,865	24,554
% Change Gross written premiums	22.70%		25.00%
% Change Ceded written premiums	(1.10%)		(4.90%)
% Change Net written premiums	33.40%		39.00%
% Change Net earned premiums	39.40%		46.10%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 32,901	30,694	$ 91,988	105,400
Ceded written premiums	(1,502)	(724)	(6,091)	(5,240)
Net written premiums	31,399	29,970	85,897	100,160
Net earned premiums	$ 37,775	$ 41,838	$ 75,785	$ 81,354
% Change Gross written premiums	7.20%		(12.70%)
% Change Ceded written premiums	107.50%		16.20%
% Change Net written premiums	4.80%		(14.20%)
% Change Net earned premiums	(9.70%)		(6.80%)