Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 497,236	$ 452,179	$ 992,460	$ 902,484
Ceded Written Premiums	(117,944)	(118,897)	(219,906)	(232,039)
Net Written Premiums	379,292	333,282	772,554	670,445
Net Earned Premiums	$ 331,015	293,835	$ 653,642	579,966
% Change Gross Written Premiums	10.00%		10.00%
% Change Ceded Written Premiums	(0.80%)		(5.20%)
% Change Net Written Premiums	13.80%		15.20%
% Change Net Earned Premiums	12.70%		12.70%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 288,478	257,186	$ 528,406	494,791
Ceded Written Premiums	(76,144)	(73,857)	(142,725)	(135,344)
Net Written Premiums	212,334	183,329	385,681	359,447
Net Earned Premiums	$ 179,746	167,087	$ 352,659	331,091
% Change Gross Written Premiums	12.20%		6.80%
% Change Ceded Written Premiums	3.10%		5.50%
% Change Net Written Premiums	15.80%		7.30%
% Change Net Earned Premiums	7.60%		6.50%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 38,830	41,687	$ 80,554	82,637
Ceded Written Premiums	(16,918)	(19,451)	(34,398)	(36,971)
Net Written Premiums	21,912	22,236	46,156	45,666
Net Earned Premiums	$ 21,177	21,812	$ 42,269	44,506
% Change Gross Written Premiums	(6.90%)		(2.50%)
% Change Ceded Written Premiums	(13.00%)		(7.00%)
% Change Net Written Premiums	(1.50%)		1.10%
% Change Net Earned Premiums	(2.90%)		(5.00%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 217,984	187,492	$ 386,177	358,126
Ceded Written Premiums	(54,835)	(51,147)	(99,747)	(89,345)
Net Written Premiums	163,149	136,345	286,430	268,781
Net Earned Premiums	$ 132,630	121,226	$ 260,220	240,349
% Change Gross Written Premiums	16.30%		7.80%
% Change Ceded Written Premiums	7.20%		11.60%
% Change Net Written Premiums	19.70%		6.60%
% Change Net Earned Premiums	9.40%		8.30%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 31,664	28,007	$ 61,675	54,028
Ceded Written Premiums	(4,391)	(3,259)	(8,580)	(9,028)
Net Written Premiums	27,273	24,748	53,095	45,000
Net Earned Premiums	$ 25,939	24,049	$ 50,170	46,236
% Change Gross Written Premiums	13.10%		14.20%
% Change Ceded Written Premiums	34.70%		(5.00%)
% Change Net Written Premiums	10.20%		18.00%
% Change Net Earned Premiums	7.90%		8.50%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 132,523	131,193	$ 260,395	273,053
Ceded Written Premiums	(39,882)	(43,809)	(68,770)	(90,402)
Net Written Premiums	92,641	87,384	191,625	182,651
Net Earned Premiums	$ 92,071	82,100	$ 185,281	166,186
% Change Gross Written Premiums	1.00%		(4.60%)
% Change Ceded Written Premiums	(9.00%)		(23.90%)
% Change Net Written Premiums	6.00%		4.90%
% Change Net Earned Premiums	12.10%		11.50%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 40,714	49,597	$ 97,192	118,430
Ceded Written Premiums	(11,523)	(12,536)	(19,577)	(23,462)
Net Written Premiums	29,191	37,061	77,615	94,968
Net Earned Premiums	$ 37,197	39,525	$ 76,476	77,020
% Change Gross Written Premiums	(17.90%)		(17.90%)
% Change Ceded Written Premiums	(8.10%)		(16.60%)
% Change Net Written Premiums	(21.20%)		(18.30%)
% Change Net Earned Premiums	(5.90%)		(0.70%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 46,579	46,663	$ 80,539	87,031
Ceded Written Premiums	(17,705)	(20,362)	(31,372)	(50,008)
Net Written Premiums	28,874	26,301	49,167	37,023
Net Earned Premiums	$ 23,715	23,337	$ 45,484	45,517
% Change Gross Written Premiums	(0.20%)		(7.50%)
% Change Ceded Written Premiums	(13.10%)		(37.30%)
% Change Net Written Premiums	9.80%		32.80%
% Change Net Earned Premiums	1.60%		(0.10%)
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 45,230	34,933	$ 82,664	67,592
Ceded Written Premiums	(10,654)	(10,911)	(17,821)	(16,932)
Net Written Premiums	34,576	24,022	64,843	50,660
Net Earned Premiums	$ 31,159	19,238	$ 63,321	43,649
% Change Gross Written Premiums	29.50%		22.30%
% Change Ceded Written Premiums	(2.40%)		5.30%
% Change Net Written Premiums	43.90%		28.00%
% Change Net Earned Premiums	62.00%		45.10%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 76,235	63,800	$ 203,659	134,640
Ceded Written Premiums	(1,918)	(1,231)	(8,411)	(6,293)
Net Written Premiums	74,317	62,569	195,248	128,347
Net Earned Premiums	$ 59,198	$ 44,648	$ 115,702	$ 82,689
% Change Gross Written Premiums	19.50%		51.30%
% Change Ceded Written Premiums	55.80%		33.70%
% Change Net Written Premiums	18.80%		52.10%
% Change Net Earned Premiums	32.60%		39.90%