Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 450,305	$ 413,877
Ceded written premiums	(113,142)	(94,057)
Net written premiums	337,163	319,820
Net earned premiums	$ 286,131	264,358
% Change Gross written premiums	8.80%
% Change Ceded written premiums	20.30%
% Change Net written premiums	5.40%
% Change Net earned premiums	8.20%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 237,605	210,647
Ceded written premiums	(61,487)	(54,497)
Net written premiums	176,118	156,150
Net earned premiums	$ 164,004	148,340
% Change Gross written premiums	12.80%
% Change Ceded written premiums	12.80%
% Change Net written premiums	12.80%
% Change Net earned premiums	10.60%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 40,950	43,163
Ceded written premiums	(17,520)	(17,647)
Net written premiums	23,430	25,516
Net earned premiums	$ 22,694	23,250
% Change Gross written premiums	(5.10%)
% Change Ceded written premiums	(0.70%)
% Change Net written premiums	(8.20%)
% Change Net earned premiums	(2.40%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 170,634	141,278
Ceded written premiums	(38,198)	(30,507)
Net written premiums	132,436	110,771
Net earned premiums	$ 119,123	109,159
% Change Gross written premiums	20.80%
% Change Ceded written premiums	25.20%
% Change Net written premiums	19.60%
% Change Net earned premiums	9.10%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 26,021	26,206
Ceded written premiums	(5,769)	(6,343)
Net written premiums	20,252	19,863
Net earned premiums	$ 22,187	15,931
% Change Gross written premiums	(0.70%)
% Change Ceded written premiums	(9.00%)
% Change Net written premiums	2.00%
% Change Net earned premiums	39.30%
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 141,860	144,143
Ceded written premiums	(46,593)	(34,971)
Net written premiums	95,267	109,172
Net earned premiums	$ 84,086	78,008
% Change Gross written premiums	(1.60%)
% Change Ceded written premiums	33.20%
% Change Net written premiums	(12.70%)
% Change Net earned premiums	7.80%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 68,833	71,948
Ceded written premiums	(10,926)	(11,089)
Net written premiums	57,907	60,859
Net earned premiums	$ 37,495	38,856
% Change Gross written premiums	(4.30%)
% Change Ceded written premiums	(1.50%)
% Change Net written premiums	(4.90%)
% Change Net earned premiums	(3.50%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 40,368	44,046
Ceded written premiums	(29,646)	(17,229)
Net written premiums	10,722	26,817
Net earned premiums	$ 22,180	21,209
% Change Gross written premiums	(8.40%)
% Change Ceded written premiums	72.10%
% Change Net written premiums	(60.00%)
% Change Net earned premiums	4.60%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 32,659	28,149
Ceded written premiums	(6,021)	(6,653)
Net written premiums	26,638	21,496
Net earned premiums	$ 24,411	17,943
% Change Gross written premiums	16.00%
% Change Ceded written premiums	(9.50%)
% Change Net written premiums	23.90%
% Change Net earned premiums	36.00%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 70,840	59,087
Ceded written premiums	(5,062)	(4,589)
Net written premiums	65,778	54,498
Net earned premiums	$ 38,041	$ 38,010
% Change Gross written premiums	19.90%
% Change Ceded written premiums	10.30%
% Change Net written premiums	20.70%
% Change Net earned premiums	0.10%