Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Gross written premiums	$ 354,062	$ 327,469	$ 1,129,993	$ 1,099,054
Ceded written premiums	(102,123)	(99,052)	(330,852)	(326,923)
Net written premiums	251,939	228,417	799,141	772,131
Net earned premiums	$ 254,082	245,367	$ 732,542	701,723
% Change Gross written premiums	8.10%		2.80%
% Change Ceded written premiums	3.10%		1.20%
% Change Net written premiums	10.30%		3.50%
% Change Net earned premiums	3.60%		4.40%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 218,783	199,723	$ 658,097	604,774
Ceded written premiums	(63,753)	(61,654)	(211,365)	(186,724)
Net written premiums	155,030	138,069	446,732	418,050
Net earned premiums	$ 142,409	130,420	$ 410,804	370,864
% Change Gross written premiums	9.50%		8.80%
% Change Ceded written premiums	3.40%		13.20%
% Change Net written premiums	12.30%		6.90%
% Change Net earned premiums	9.20%		10.80%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 35,529	34,576	$ 118,167	120,149
Ceded written premiums	(13,865)	(13,338)	(45,177)	(34,095)
Net written premiums	21,664	21,238	72,990	86,054
Net earned premiums	$ 24,292	27,207	$ 74,193	81,920
% Change Gross written premiums	2.80%		(1.60%)
% Change Ceded written premiums	4.00%		32.50%
% Change Net written premiums	2.00%		(15.20%)
% Change Net earned premiums	(10.70%)		(9.40%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 154,009	138,181	$ 458,371	400,437
Ceded written premiums	(34,902)	(41,180)	(123,765)	(127,788)
Net written premiums	119,107	97,001	334,606	272,649
Net earned premiums	$ 104,473	82,635	$ 291,648	224,151
% Change Gross written premiums	11.50%		14.50%
% Change Ceded written premiums	(15.20%)		(3.10%)
% Change Net written premiums	22.80%		22.70%
% Change Net earned premiums	26.40%		30.10%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 29,245	26,966	$ 81,559	84,188
Ceded written premiums	(14,986)	(7,136)	(42,423)	(24,841)
Net written premiums	14,259	19,830	39,136	59,347
Net earned premiums	$ 13,644	20,578	$ 44,963	64,793
% Change Gross written premiums	8.50%		(3.10%)
% Change Ceded written premiums	110.00%		70.80%
% Change Net written premiums	(28.10%)		(34.10%)
% Change Net earned premiums	(33.70%)		(30.60%)
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 98,788	94,161	$ 330,005	334,085
Ceded written premiums	(37,804)	(36,607)	(113,681)	(135,534)
Net written premiums	60,984	57,554	216,324	198,551
Net earned premiums	$ 68,533	66,278	$ 197,244	187,993
% Change Gross written premiums	4.90%		(1.20%)
% Change Ceded written premiums	3.30%		(16.10%)
% Change Net written premiums	6.00%		9.00%
% Change Net earned premiums	3.40%		4.90%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 41,002	41,593	$ 149,844	151,107
Ceded written premiums	(10,985)	(6,924)	(30,933)	(36,351)
Net written premiums	30,017	34,669	118,911	114,756
Net earned premiums	$ 36,792	40,207	$ 114,716	112,428
% Change Gross written premiums	(1.40%)		(0.80%)
% Change Ceded written premiums	58.70%		(14.90%)
% Change Net written premiums	(13.40%)		3.60%
% Change Net earned premiums	(8.50%)		2.00%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 37,449	35,399	$ 112,014	128,954
Ceded written premiums	(20,034)	(24,267)	(60,676)	(80,015)
Net written premiums	17,415	11,132	51,338	48,939
Net earned premiums	$ 16,879	14,300	$ 43,112	47,207
% Change Gross written premiums	5.80%		(13.10%)
% Change Ceded written premiums	(17.40%)		(24.20%)
% Change Net written premiums	56.40%		4.90%
% Change Net earned premiums	18.00%		(8.70%)
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 20,337	17,169	$ 68,147	54,024
Ceded written premiums	(6,785)	(5,416)	(22,072)	(19,168)
Net written premiums	13,552	11,753	46,075	34,856
Net earned premiums	$ 14,862	11,771	$ 39,416	28,358
% Change Gross written premiums	18.50%		26.10%
% Change Ceded written premiums	25.30%		15.20%
% Change Net written premiums	15.30%		32.20%
% Change Net earned premiums	26.30%		39.00%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 36,491	33,585	$ 141,891	160,195
Ceded written premiums	(566)	(791)	(5,806)	(4,665)
Net written premiums	35,925	32,794	136,085	155,530
Net earned premiums	$ 43,140	$ 48,669	$ 124,494	$ 142,866
% Change Gross written premiums	8.70%		(11.40%)
% Change Ceded written premiums	(28.40%)		24.50%
% Change Net written premiums	9.50%		(12.50%)
% Change Net earned premiums	(11.40%)		(12.90%)