Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 402,038	$ 374,930	$ 1,304,522	$ 1,201,372
Ceded Written Premiums	(106,022)	(97,929)	(338,061)	(298,016)
Net Written Premiums	296,016	277,001	966,461	903,356
Net Earned Premiums	$ 301,355	284,010	$ 881,321	816,360
% Change Gross Written Premiums	7.20%		8.60%
% Change Ceded Written Premiums	8.30%		13.40%
% Change Net Written Premiums	6.90%		7.00%
% Change Net Earned Premiums	6.10%		8.00%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 235,052	234,647	$ 729,843	689,218
Ceded Written Premiums	(64,328)	(61,359)	(199,672)	(178,474)
Net Written Premiums	170,724	173,288	530,171	510,744
Net Earned Premiums	$ 169,150	161,722	$ 500,241	462,446
% Change Gross Written Premiums	0.20%		5.90%
% Change Ceded Written Premiums	4.80%		11.90%
% Change Net Written Premiums	(1.50%)		3.80%
% Change Net Earned Premiums	4.60%		8.20%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 36,403	40,271	$ 119,040	129,621
Ceded Written Premiums	(18,859)	(17,636)	(55,830)	(53,823)
Net Written Premiums	17,544	22,635	63,210	75,798
Net Earned Premiums	$ 20,129	25,465	$ 64,635	73,636
% Change Gross Written Premiums	(9.60%)		(8.20%)
% Change Ceded Written Premiums	6.90%		3.70%
% Change Net Written Premiums	(22.50%)		(16.60%)
% Change Net Earned Premiums	(21.00%)		(12.20%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 166,097	161,600	$ 524,223	471,532
Ceded Written Premiums	(39,952)	(36,011)	(129,297)	(103,286)
Net Written Premiums	126,145	125,589	394,926	368,246
Net Earned Premiums	$ 125,931	115,971	$ 366,280	335,342
% Change Gross Written Premiums	2.80%		11.20%
% Change Ceded Written Premiums	10.90%		25.20%
% Change Net Written Premiums	0.40%		7.20%
% Change Net Earned Premiums	8.60%		9.20%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 32,552	32,776	$ 86,580	88,065
Ceded Written Premiums	(5,517)	(7,712)	(14,545)	(21,365)
Net Written Premiums	27,035	25,064	72,035	66,700
Net Earned Premiums	$ 23,090	20,286	$ 69,326	53,468
% Change Gross Written Premiums	(0.70%)		(1.70%)
% Change Ceded Written Premiums	(28.50%)		(31.90%)
% Change Net Written Premiums	7.90%		8.00%
% Change Net Earned Premiums	13.80%		29.70%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 113,601	104,484	$ 386,654	384,367
Ceded Written Premiums	(38,966)	(35,194)	(129,368)	(112,075)
Net Written Premiums	74,635	69,290	257,286	272,292
Net Earned Premiums	$ 84,407	79,761	$ 250,593	235,602
% Change Gross Written Premiums	8.70%		0.60%
% Change Ceded Written Premiums	10.70%		15.40%
% Change Net Written Premiums	7.70%		(5.50%)
% Change Net Earned Premiums	5.80%		6.40%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 41,689	38,453	$ 160,119	151,548
Ceded Written Premiums	(9,948)	(12,631)	(33,410)	(33,274)
Net Written Premiums	31,741	25,822	126,709	118,274
Net Earned Premiums	$ 39,038	34,794	$ 116,058	109,746
% Change Gross Written Premiums	8.40%		5.70%
% Change Ceded Written Premiums	(21.20%)		0.40%
% Change Net Written Premiums	22.90%		7.10%
% Change Net Earned Premiums	12.20%		5.80%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 37,415	38,077	$ 124,446	145,125
Ceded Written Premiums	(19,772)	(15,331)	(69,780)	(57,025)
Net Written Premiums	17,643	22,746	54,666	88,100
Net Earned Premiums	$ 21,775	25,584	$ 67,292	70,608
% Change Gross Written Premiums	(1.70%)		(14.20%)
% Change Ceded Written Premiums	29.00%		22.40%
% Change Net Written Premiums	(22.40%)		(37.90%)
% Change Net Earned Premiums	(14.90%)		(4.70%)
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 34,497	27,954	$ 102,089	87,694
Ceded Written Premiums	(9,246)	(7,232)	(26,178)	(21,776)
Net Written Premiums	25,251	20,722	75,911	65,918
Net Earned Premiums	$ 23,594	19,383	$ 67,243	55,248
% Change Gross Written Premiums	23.40%		16.40%
% Change Ceded Written Premiums	27.80%		20.20%
% Change Net Written Premiums	21.90%		15.20%
% Change Net Earned Premiums	21.70%		21.70%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 53,385	35,799	$ 188,025	127,787
Ceded Written Premiums	(2,728)	(1,376)	(9,021)	(7,467)
Net Written Premiums	50,657	34,423	179,004	120,320
Net Earned Premiums	$ 47,798	$ 42,527	$ 130,487	$ 118,312
% Change Gross Written Premiums	49.10%		47.10%
% Change Ceded Written Premiums	98.30%		20.80%
% Change Net Written Premiums	47.20%		48.80%
% Change Net Earned Premiums	12.40%		10.30%