Additional Financial Data by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Gross written premiums	$ 379,471	$ 348,795	$ 775,931	$ 771,585
Ceded written premiums	(121,227)	(116,931)	(228,729)	(227,871)
Net written premiums	258,244	231,864	547,202	543,714
Net earned premiums	$ 242,328	231,084	$ 478,460	456,356
% Change Gross written premiums	8.80%		0.60%
% Change Ceded written premiums	3.70%		0.40%
% Change Net written premiums	11.40%		0.60%
% Change Net earned premiums	4.90%		4.80%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 239,987	206,703	$ 439,314	405,051
Ceded written premiums	(78,882)	(60,004)	(147,612)	(125,070)
Net written premiums	161,105	146,699	291,702	279,981
Net earned premiums	$ 137,304	125,254	$ 268,395	240,444
% Change Gross written premiums	16.10%		8.50%
% Change Ceded written premiums	31.50%		18.00%
% Change Net written premiums	9.80%		4.20%
% Change Net earned premiums	9.60%		11.60%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 41,803	42,548	$ 82,638	85,573
Ceded written premiums	(15,026)	(11,587)	(31,312)	(20,757)
Net written premiums	26,777	30,961	51,326	64,816
Net earned premiums	$ 25,977	26,727	$ 49,901	54,713
% Change Gross written premiums	(1.70%)		(3.40%)
% Change Ceded written premiums	29.70%		50.80%
% Change Net written premiums	(13.50%)		(20.80%)
% Change Net earned premiums	(2.80%)		(8.80%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 170,216	137,137	$ 304,362	262,256
Ceded written premiums	(49,305)	(40,452)	(88,863)	(86,608)
Net written premiums	120,911	96,685	215,499	175,648
Net earned premiums	$ 96,440	76,585	$ 187,175	141,516
% Change Gross written premiums	24.10%		16.10%
% Change Ceded written premiums	21.90%		2.60%
% Change Net written premiums	25.10%		22.70%
% Change Net earned premiums	25.90%		32.30%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 27,968	27,018	$ 52,314	57,222
Ceded written premiums	(14,551)	(7,965)	(27,437)	(17,705)
Net written premiums	13,417	19,053	24,877	39,517
Net earned premiums	$ 14,887	21,942	$ 31,319	44,215
% Change Gross written premiums	3.50%		(8.60%)
% Change Ceded written premiums	82.70%		55.00%
% Change Net written premiums	(29.60%)		(37.00%)
% Change Net earned premiums	(32.20%)		(29.20%)
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 108,790	120,732	$ 231,217	239,924
Ceded written premiums	(41,621)	(56,345)	(75,877)	(98,927)
Net written premiums	67,169	64,387	155,340	140,997
Net earned premiums	$ 63,186	59,661	$ 128,711	121,715
% Change Gross written premiums	(9.90%)		(3.60%)
% Change Ceded written premiums	(26.10%)		(23.30%)
% Change Net written premiums	4.30%		10.20%
% Change Net earned premiums	5.90%		5.70%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums	$ 41,432	47,532	$ 108,842	109,514
Ceded written premiums	(9,126)	(17,033)	(19,948)	(29,427)
Net written premiums	32,306	30,499	88,894	80,087
Net earned premiums	$ 40,132	34,177	$ 77,924	72,221
% Change Gross written premiums	(12.80%)		(0.60%)
% Change Ceded written premiums	(46.40%)		(32.20%)
% Change Net written premiums	5.90%		11.00%
% Change Net earned premiums	17.40%		7.90%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	$ 41,615	51,305	$ 74,565	93,555
Ceded written premiums	(24,513)	(31,284)	(40,642)	(55,748)
Net written premiums	17,102	20,021	33,923	37,807
Net earned premiums	$ 10,199	16,466	$ 26,233	32,907
% Change Gross written premiums	(18.90%)		(20.30%)
% Change Ceded written premiums	(21.60%)		(27.10%)
% Change Net written premiums	(14.60%)		(10.30%)
% Change Net earned premiums	(38.10%)		(20.30%)
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	$ 25,743	21,895	$ 47,810	36,855
Ceded written premiums	(7,982)	(8,028)	(15,287)	(13,752)
Net written premiums	17,761	13,867	32,523	23,103
Net earned premiums	$ 12,855	9,018	$ 24,554	16,587
% Change Gross written premiums	17.60%		29.70%
% Change Ceded written premiums	(0.60%)		11.20%
% Change Net written premiums	28.10%		40.80%
% Change Net earned premiums	42.50%		48.00%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 30,694	21,360	$ 105,400	126,610
Ceded written premiums	(724)	(582)	(5,240)	(3,874)
Net written premiums	29,970	20,778	100,160	122,736
Net earned premiums	$ 41,838	$ 46,169	$ 81,354	$ 94,197
% Change Gross written premiums	43.70%		(16.80%)
% Change Ceded written premiums	24.40%		35.30%
% Change Net written premiums	44.20%		(18.40%)
% Change Net earned premiums	(9.40%)		(13.60%)