Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 495,224	$ 450,305
Ceded Written Premiums	(101,962)	(113,142)
Net Written Premiums	393,262	337,163
Net Earned Premiums	$ 322,627	286,131
% Change Gross Written Premiums	10.00%
% Change Ceded Written Premiums	(9.90%)
% Change Net Written Premiums	16.60%
% Change Net Earned Premiums	12.80%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 239,928	237,605
Ceded Written Premiums	(66,581)	(61,487)
Net Written Premiums	173,347	176,118
Net Earned Premiums	$ 172,913	164,004
% Change Gross Written Premiums	1.00%
% Change Ceded Written Premiums	8.30%
% Change Net Written Premiums	(1.60%)
% Change Net Earned Premiums	5.40%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 41,724	40,950
Ceded Written Premiums	(17,480)	(17,520)
Net Written Premiums	24,244	23,430
Net Earned Premiums	$ 21,092	22,694
% Change Gross Written Premiums	1.90%
% Change Ceded Written Premiums	(0.20%)
% Change Net Written Premiums	3.50%
% Change Net Earned Premiums	(7.10%)
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 168,193	170,634
Ceded Written Premiums	(44,912)	(38,198)
Net Written Premiums	123,281	132,436
Net Earned Premiums	$ 127,590	119,123
% Change Gross Written Premiums	(1.40%)
% Change Ceded Written Premiums	17.60%
% Change Net Written Premiums	(6.90%)
% Change Net Earned Premiums	7.10%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 30,011	26,021
Ceded Written Premiums	(4,189)	(5,769)
Net Written Premiums	25,822	20,252
Net Earned Premiums	$ 24,231	22,187
% Change Gross Written Premiums	15.30%
% Change Ceded Written Premiums	(27.40%)
% Change Net Written Premiums	27.50%
% Change Net Earned Premiums	9.20%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 127,872	141,860
Ceded Written Premiums	(28,888)	(46,593)
Net Written Premiums	98,984	95,267
Net Earned Premiums	$ 93,210	84,086
% Change Gross Written Premiums	(9.90%)
% Change Ceded Written Premiums	(38.00%)
% Change Net Written Premiums	3.90%
% Change Net Earned Premiums	10.90%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 56,478	68,833
Ceded Written Premiums	(8,054)	(10,926)
Net Written Premiums	48,424	57,907
Net Earned Premiums	$ 39,279	37,495
% Change Gross Written Premiums	(17.90%)
% Change Ceded Written Premiums	(26.30%)
% Change Net Written Premiums	(16.40%)
% Change Net Earned Premiums	4.80%
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 33,960	40,368
Ceded Written Premiums	(13,667)	(29,646)
Net Written Premiums	20,293	10,722
Net Earned Premiums	$ 21,769	22,180
% Change Gross Written Premiums	(15.90%)
% Change Ceded Written Premiums	(53.90%)
% Change Net Written Premiums	89.30%
% Change Net Earned Premiums	(1.90%)
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 37,434	32,659
Ceded Written Premiums	(7,167)	(6,021)
Net Written Premiums	30,267	26,638
Net Earned Premiums	$ 32,162	24,411
% Change Gross Written Premiums	14.60%
% Change Ceded Written Premiums	19.00%
% Change Net Written Premiums	13.60%
% Change Net Earned Premiums	31.80%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 127,424	70,840
Ceded Written Premiums	(6,493)	(5,062)
Net Written Premiums	120,931	65,778
Net Earned Premiums	$ 56,504	$ 38,041
% Change Gross Written Premiums	79.90%
% Change Ceded Written Premiums	28.30%
% Change Net Written Premiums	83.80%
% Change Net Earned Premiums	48.50%