Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 374,930	$ 354,062	$ 1,201,372	$ 1,129,993
Ceded written premiums	(97,929)	(102,123)	(298,016)	(330,852)
Net written premiums	277,001	251,939	903,356	799,141
Net earned premiums	$ 284,010	254,082	$ 816,360	732,542
% Change Gross written premiums	5.90%		6.30%
% Change Ceded written premiums	(4.10%)		(9.90%)
% Change Net written premiums	9.90%		13.00%
% Change Net earned premiums	11.80%		11.40%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 234,647	218,783	$ 689,218	658,097
Ceded written premiums	(61,359)	(63,753)	(178,474)	(211,365)
Net written premiums	173,288	155,030	510,744	446,732
Net earned premiums	$ 161,722	142,409	$ 462,446	410,804
% Change Gross written premiums	7.30%		4.70%
% Change Ceded written premiums	(3.80%)		(15.60%)
% Change Net written premiums	11.80%		14.30%
% Change Net earned premiums	13.60%		12.60%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 40,271	35,529	$ 129,621	118,167
Ceded written premiums	(17,636)	(13,865)	(53,823)	(45,177)
Net written premiums	22,635	21,664	75,798	72,990
Net earned premiums	$ 25,465	24,292	$ 73,636	74,193
% Change Gross written premiums	13.30%		9.70%
% Change Ceded written premiums	27.20%		19.10%
% Change Net written premiums	4.50%		3.80%
% Change Net earned premiums	4.80%		(0.70%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 161,600	154,009	$ 471,532	458,371
Ceded written premiums	(36,011)	(34,902)	(103,286)	(123,765)
Net written premiums	125,589	119,107	368,246	334,606
Net earned premiums	$ 115,971	104,473	$ 335,342	291,648
% Change Gross written premiums	4.90%		2.90%
% Change Ceded written premiums	3.20%		(16.50%)
% Change Net written premiums	5.40%		10.10%
% Change Net earned premiums	11.00%		15.00%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 32,776	29,245	$ 88,065	81,559
Ceded written premiums	(7,712)	(14,986)	(21,365)	(42,423)
Net written premiums	25,064	14,259	66,700	39,136
Net earned premiums	$ 20,286	13,644	$ 53,468	44,963
% Change Gross written premiums	12.10%		8.00%
% Change Ceded written premiums	(48.50%)		(49.60%)
% Change Net written premiums	75.80%		70.40%
% Change Net earned premiums	48.70%		18.90%
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 104,484	98,788	$ 384,367	330,005
Ceded written premiums	(35,194)	(37,804)	(112,075)	(113,681)
Net written premiums	69,290	60,984	272,292	216,324
Net earned premiums	$ 79,761	68,533	$ 235,602	197,244
% Change Gross written premiums	5.80%		16.50%
% Change Ceded written premiums	(6.90%)		(1.40%)
% Change Net written premiums	13.60%		25.90%
% Change Net earned premiums	16.40%		19.40%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 38,453	41,002	$ 151,548	149,844
Ceded written premiums	(12,631)	(10,985)	(33,274)	(30,933)
Net written premiums	25,822	30,017	118,274	118,911
Net earned premiums	$ 34,794	36,792	$ 109,746	114,716
% Change Gross written premiums	(6.20%)		1.10%
% Change Ceded written premiums	15.00%		7.60%
% Change Net written premiums	(14.00%)		(0.50%)
% Change Net earned premiums	(5.40%)		(4.30%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 38,077	37,449	$ 145,125	112,014
Ceded written premiums	(15,331)	(20,034)	(57,025)	(60,676)
Net written premiums	22,746	17,415	88,100	51,338
Net earned premiums	$ 25,584	16,879	$ 70,608	43,112
% Change Gross written premiums	1.70%		29.60%
% Change Ceded written premiums	(23.50%)		(6.00%)
% Change Net written premiums	30.60%		71.60%
% Change Net earned premiums	51.60%		63.80%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 27,954	20,337	$ 87,694	68,147
Ceded written premiums	(7,232)	(6,785)	(21,776)	(22,072)
Net written premiums	20,722	13,552	65,918	46,075
Net earned premiums	$ 19,383	14,862	$ 55,248	39,416
% Change Gross written premiums	37.50%		28.70%
% Change Ceded written premiums	6.60%		(1.30%)
% Change Net written premiums	52.90%		43.10%
% Change Net earned premiums	30.40%		40.20%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 35,799	36,491	$ 127,787	141,891
Ceded written premiums	(1,376)	(566)	(7,467)	(5,806)
Net written premiums	34,423	35,925	120,320	136,085
Net earned premiums	$ 42,527	$ 43,140	$ 118,312	$ 124,494
% Change Gross written premiums	(1.90%)		(9.90%)
% Change Ceded written premiums	143.20%		28.60%
% Change Net written premiums	(4.20%)		(11.60%)
% Change Net earned premiums	(1.40%)		(5.00%)