Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 413,877	$ 396,460
Ceded written premiums	(94,057)	(107,502)
Net written premiums	319,820	288,958
Net earned premiums	$ 264,358	236,132
% Change Gross written premiums	4.40%
% Change Ceded written premiums	(12.50%)
% Change Net written premiums	10.70%
% Change Net earned premiums	12.00%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 210,647	199,327
Ceded written premiums	(54,497)	(68,730)
Net written premiums	156,150	130,597
Net earned premiums	$ 148,340	131,091
% Change Gross written premiums	5.70%
% Change Ceded written premiums	(20.70%)
% Change Net written premiums	19.60%
% Change Net earned premiums	13.20%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 43,163	40,835
Ceded written premiums	(17,647)	(16,286)
Net written premiums	25,516	24,549
Net earned premiums	$ 23,250	23,924
% Change Gross written premiums	5.70%
% Change Ceded written premiums	8.40%
% Change Net written premiums	3.90%
% Change Net earned premiums	(2.80%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 141,278	134,146
Ceded written premiums	(30,507)	(39,558)
Net written premiums	110,771	94,588
Net earned premiums	$ 109,159	90,735
% Change Gross written premiums	5.30%
% Change Ceded written premiums	(22.90%)
% Change Net written premiums	17.10%
% Change Net earned premiums	20.30%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 26,206	24,346
Ceded written premiums	(6,343)	(12,886)
Net written premiums	19,863	11,460
Net earned premiums	$ 15,931	16,432
% Change Gross written premiums	7.60%
% Change Ceded written premiums	(50.80%)
% Change Net written premiums	73.30%
% Change Net earned premiums	(3.00%)
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 144,143	122,427
Ceded written premiums	(34,971)	(34,256)
Net written premiums	109,172	88,171
Net earned premiums	$ 78,008	65,525
% Change Gross written premiums	17.70%
% Change Ceded written premiums	2.10%
% Change Net written premiums	23.80%
% Change Net earned premiums	19.10%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 71,948	67,410
Ceded written premiums	(11,089)	(10,822)
Net written premiums	60,859	56,588
Net earned premiums	$ 38,856	37,792
% Change Gross written premiums	6.70%
% Change Ceded written premiums	2.50%
% Change Net written premiums	7.50%
% Change Net earned premiums	2.80%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 44,046	32,950
Ceded written premiums	(17,229)	(16,129)
Net written premiums	26,817	16,821
Net earned premiums	$ 21,209	16,034
% Change Gross written premiums	33.70%
% Change Ceded written premiums	6.80%
% Change Net written premiums	59.40%
% Change Net earned premiums	32.30%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 28,149	22,067
Ceded written premiums	(6,653)	(7,305)
Net written premiums	21,496	14,762
Net earned premiums	$ 17,943	11,699
% Change Gross written premiums	27.60%
% Change Ceded written premiums	(8.90%)
% Change Net written premiums	45.60%
% Change Net earned premiums	53.40%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 59,087	74,706
Ceded written premiums	(4,589)	(4,516)
Net written premiums	54,498	70,190
Net earned premiums	$ 38,010	$ 39,516
% Change Gross written premiums	(20.90%)
% Change Ceded written premiums	1.60%
% Change Net written premiums	(22.40%)
% Change Net earned premiums	(3.80%)