Revenue by Operating Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 580,976	$ 495,224
Ceded Written Premiums	(119,309)	(101,962)
Net Written Premiums	461,667	393,262
Net Earned Premiums	$ 377,189	322,627
% Change Gross Written Premiums	17.30%
% Change Ceded Written Premiums	17.00%
% Change Net Written Premiums	17.40%
% Change Net Earned Premiums	16.90%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 291,967	239,928
Ceded Written Premiums	(73,795)	(66,581)
Net Written Premiums	218,172	173,347
Net Earned Premiums	$ 202,037	172,913
% Change Gross Written Premiums	21.70%
% Change Ceded Written Premiums	10.80%
% Change Net Written Premiums	25.90%
% Change Net Earned Premiums	16.80%
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 47,801	41,724
Ceded Written Premiums	(15,077)	(17,480)
Net Written Premiums	32,724	24,244
Net Earned Premiums	$ 23,357	21,092
% Change Gross Written Premiums	14.60%
% Change Ceded Written Premiums	(13.70%)
% Change Net Written Premiums	35.00%
% Change Net Earned Premiums	10.70%
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 207,802	168,193
Ceded Written Premiums	(53,544)	(44,912)
Net Written Premiums	154,258	123,281
Net Earned Premiums	$ 151,171	127,590
% Change Gross Written Premiums	23.50%
% Change Ceded Written Premiums	19.20%
% Change Net Written Premiums	25.10%
% Change Net Earned Premiums	18.50%
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 36,364	30,011
Ceded Written Premiums	(5,174)	(4,189)
Net Written Premiums	31,190	25,822
Net Earned Premiums	$ 27,509	24,231
% Change Gross Written Premiums	21.20%
% Change Ceded Written Premiums	23.50%
% Change Net Written Premiums	20.80%
% Change Net Earned Premiums	13.50%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 156,129	127,872
Ceded Written Premiums	(37,731)	(28,888)
Net Written Premiums	118,398	98,984
Net Earned Premiums	$ 108,913	93,210
% Change Gross Written Premiums	22.10%
% Change Ceded Written Premiums	30.60%
% Change Net Written Premiums	19.60%
% Change Net Earned Premiums	16.80%
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 54,861	56,478
Ceded Written Premiums	(11,236)	(8,054)
Net Written Premiums	43,625	48,424
Net Earned Premiums	$ 35,744	39,279
% Change Gross Written Premiums	(2.90%)
% Change Ceded Written Premiums	39.50%
% Change Net Written Premiums	(9.90%)
% Change Net Earned Premiums	(9.00%)
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 58,567	33,960
Ceded Written Premiums	(14,037)	(13,667)
Net Written Premiums	44,530	20,293
Net Earned Premiums	$ 36,033	21,769
% Change Gross Written Premiums	72.50%
% Change Ceded Written Premiums	2.70%
% Change Net Written Premiums	119.40%
% Change Net Earned Premiums	65.50%
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 42,701	37,434
Ceded Written Premiums	(12,458)	(7,167)
Net Written Premiums	30,243	30,267
Net Earned Premiums	$ 37,136	32,162
% Change Gross Written Premiums	14.10%
% Change Ceded Written Premiums	73.80%
% Change Net Written Premiums	(0.10%)
% Change Net Earned Premiums	15.50%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums	$ 132,880	127,424
Ceded Written Premiums	(7,783)	(6,493)
Net Written Premiums	125,097	120,931
Net Earned Premiums	$ 66,239	$ 56,504
% Change Gross Written Premiums	4.30%
% Change Ceded Written Premiums	19.90%
% Change Net Written Premiums	3.40%
% Change Net Earned Premiums	17.20%