Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 455,943	$ 402,038	$ 1,448,403	$ 1,304,522
Ceded written premiums	(103,531)	(106,022)	(323,437)	(338,061)
Net written premiums	352,412	296,016	1,124,966	966,461
Net Earned Premiums	$ 347,038	301,355	$ 1,000,680	881,321
% Change Gross Written Premiums	13.40%		11.00%
% Change Ceded Written Premiums	(2.30%)		(4.30%)
% Change Net Written Premiums	19.10%		16.40%
% Change Net Earned Premiums	15.20%		13.50%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 280,183	235,052	$ 808,589	729,843
Ceded written premiums	(72,615)	(64,328)	(215,340)	(199,672)
Net written premiums	207,568	170,724	593,249	530,171
Net Earned Premiums	$ 188,504	169,150	$ 541,163	500,241
% Change Gross Written Premiums	19.20%		10.80%
% Change Ceded Written Premiums	12.90%		7.80%
% Change Net Written Premiums	21.60%		11.90%
% Change Net Earned Premiums	11.40%		8.20%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 31,681	36,403	$ 112,235	119,040
Ceded written premiums	(14,442)	(18,859)	(48,840)	(55,830)
Net written premiums	17,239	17,544	63,395	63,210
Net Earned Premiums	$ 20,401	20,129	$ 62,670	64,635
% Change Gross Written Premiums	(13.00%)		(5.70%)
% Change Ceded Written Premiums	(23.40%)		(12.50%)
% Change Net Written Premiums	(1.70%)		0.30%
% Change Net Earned Premiums	1.40%		(3.00%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 212,911	166,097	$ 599,088	524,223
Ceded written premiums	(52,993)	(39,952)	(152,740)	(129,297)
Net written premiums	159,918	126,145	446,348	394,926
Net Earned Premiums	$ 141,650	125,931	$ 401,870	366,280
% Change Gross Written Premiums	28.20%		14.30%
% Change Ceded Written Premiums	32.60%		18.10%
% Change Net Written Premiums	26.80%		13.00%
% Change Net Earned Premiums	12.50%		9.70%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 35,591	32,552	$ 97,266	86,580
Ceded written premiums	(5,180)	(5,517)	(13,760)	(14,545)
Net written premiums	30,411	27,035	83,506	72,035
Net Earned Premiums	$ 26,453	23,090	$ 76,623	69,326
% Change Gross Written Premiums	9.30%		12.30%
% Change Ceded Written Premiums	(6.10%)		(5.40%)
% Change Net Written Premiums	12.50%		15.90%
% Change Net Earned Premiums	14.60%		10.50%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 117,376	113,601	$ 377,771	386,654
Ceded written premiums	(29,020)	(38,966)	(97,790)	(129,368)
Net written premiums	88,356	74,635	279,981	257,286
Net Earned Premiums	$ 95,226	84,407	$ 280,507	250,593
% Change Gross Written Premiums	3.30%		(2.30%)
% Change Ceded Written Premiums	(25.50%)		(24.40%)
% Change Net Written Premiums	18.40%		8.80%
% Change Net Earned Premiums	12.80%		11.90%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 32,486	41,689	$ 129,678	160,119
Ceded written premiums	(4,965)	(9,948)	(24,542)	(33,410)
Net written premiums	27,521	31,741	105,136	126,709
Net Earned Premiums	$ 38,866	39,038	$ 115,342	116,058
% Change Gross Written Premiums	(22.10%)		(19.00%)
% Change Ceded Written Premiums	(50.10%)		(26.50%)
% Change Net Written Premiums	(13.30%)		(17.00%)
% Change Net Earned Premiums	(0.40%)		(0.60%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 41,898	37,415	$ 122,437	124,446
Ceded written premiums	(13,727)	(19,772)	(45,099)	(69,780)
Net written premiums	28,171	17,643	77,338	54,666
Net Earned Premiums	$ 29,616	21,775	$ 75,100	67,292
% Change Gross Written Premiums	12.00%		(1.60%)
% Change Ceded Written Premiums	(30.60%)		(35.40%)
% Change Net Written Premiums	59.70%		41.50%
% Change Net Earned Premiums	36.00%		11.60%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 42,992	34,497	$ 125,656	102,089
Ceded written premiums	(10,328)	(9,246)	(28,149)	(26,178)
Net written premiums	32,664	25,251	97,507	75,911
Net Earned Premiums	$ 26,744	23,594	$ 90,065	67,243
% Change Gross Written Premiums	24.60%		23.10%
% Change Ceded Written Premiums	11.70%		7.50%
% Change Net Written Premiums	29.40%		28.40%
% Change Net Earned Premiums	13.30%		33.90%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 58,384	53,385	$ 262,043	188,025
Ceded written premiums	(1,896)	(2,728)	(10,307)	(9,021)
Net written premiums	56,488	50,657	251,736	179,004
Net Earned Premiums	$ 63,308	$ 47,798	$ 179,010	$ 130,487
% Change Gross Written Premiums	9.40%		39.40%
% Change Ceded Written Premiums	(30.50%)		14.30%
% Change Net Written Premiums	11.50%		40.60%
% Change Net Earned Premiums	32.40%		37.20%