Supplementary Insurance Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		$ 135,249	$ 119,660	$ 91,983
Reserve for Losses and LAE		2,515,145	2,289,727	2,202,644
Unearned Premiums		987,681	887,344	820,676
Net Earned Premiums		1,186,420	1,100,345	984,087
Net Investment Income	[1]	89,293	79,451	68,718
Losses and LAE Incurred		806,265	665,448	572,598
Amortization of Deferred Policy Acquisition Costs		184,731	165,045	129,977
Other Operating Expenses		233,230	234,096	223,516
Net Written Premiums		1,271,330	1,186,224	1,043,860
U.S.Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		54,674	50,028	37,372
Reserve for Losses and LAE		1,629,015	1,564,524	1,478,391
Unearned Premiums		575,068	519,505	476,803
Net Earned Premiums		674,665	629,308	555,836
Losses and LAE Incurred		443,353	397,860	343,497
Amortization of Deferred Policy Acquisition Costs		77,729	70,812	56,319
Other Operating Expenses		128,905	128,108	131,407
Net Written Premiums		721,300	683,568	597,006
International Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		38,988	37,385	25,451
Reserve for Losses and LAE		716,352	605,105	608,783
Unearned Premiums		260,982	252,442	221,544
Net Earned Premiums		333,792	307,416	259,960
Losses and LAE Incurred		229,601	178,284	134,702
Amortization of Deferred Policy Acquisition Costs		68,824	61,703	43,676
Other Operating Expenses		83,464	86,395	75,867
Net Written Premiums		335,852	345,967	277,942
Global Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		41,587	32,247	29,160
Reserve for Losses and LAE		169,778	120,098	115,470
Unearned Premiums		151,631	115,397	122,329
Net Earned Premiums		177,963	163,621	168,291
Losses and LAE Incurred		133,311	89,304	94,399
Amortization of Deferred Policy Acquisition Costs		39,136	34,008	32,240
Other Operating Expenses		20,861	19,593	16,242
Net Written Premiums		214,178	156,689	168,912
Corporate
Supplementary Insurance Information, by Segment [Line Items]
Net Investment Income	[1]	89,293	79,451	68,718
Amortization of Deferred Policy Acquisition Costs		$ (958)	$ (1,478)	$ (2,258)

[1] As we evaluate the underwriting results of each of our reportable segments separately from the results of our investment portfolio, we do not allocate net investment income to our reportable segments.