Supplementary Insurance Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		$ 158,080	$ 135,249	$ 119,660
Reserve for Losses and LAE		2,743,973	2,515,145	2,289,727
Unearned Premiums		1,102,735	987,681	887,344
Net Earned Premiums		1,363,223	1,186,420	1,100,345
Net Investment Income	[1]	103,113	89,293	79,451
Losses and LAE Incurred		893,779	806,265	665,448
Amortization of Deferred Policy Acquisition Costs		220,144	184,731	165,045
Other Operating Expenses		261,224	233,230	234,096
Net Written Premiums		1,471,085	1,271,330	1,186,224
U.S.Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		63,903	54,674	50,028
Reserve for Losses and LAE		1,784,339	1,629,015	1,564,524
Unearned Premiums		642,725	575,068	519,505
Net Earned Premiums		737,646	674,665	629,308
Losses and LAE Incurred		485,960	443,353	397,860
Amortization of Deferred Policy Acquisition Costs		85,127	77,729	70,812
Other Operating Expenses		142,333	128,905	128,108
Net Written Premiums		800,091	721,300	683,568
International Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		39,472	38,988	37,385
Reserve for Losses and LAE		768,301	716,352	605,105
Unearned Premiums		265,753	260,982	252,442
Net Earned Premiums		380,503	333,792	307,416
Losses and LAE Incurred		252,268	229,601	178,284
Amortization of Deferred Policy Acquisition Costs		80,526	68,824	61,703
Other Operating Expenses		93,882	83,464	86,395
Net Written Premiums		383,308	335,852	345,967
Global Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		54,705	41,587	32,247
Reserve for Losses and LAE		191,333	169,778	120,098
Unearned Premiums		194,257	151,631	115,397
Net Earned Premiums		245,074	177,963	163,621
Losses and LAE Incurred		155,551	133,311	89,304
Amortization of Deferred Policy Acquisition Costs		55,056	39,136	34,008
Other Operating Expenses		25,009	20,861	19,593
Net Written Premiums		287,686	214,178	156,689
Corporate
Supplementary Insurance Information, by Segment [Line Items]
Net Investment Income	[1]	103,113	89,293	79,451
Amortization of Deferred Policy Acquisition Costs		$ (565)	$ (958)	$ (1,478)

[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.