Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 322,627	$ 286,131
Net Losses and LAE		(186,145)	(169,600)
Commission Expenses		(54,152)	(47,844)
Other Operating Expenses		(62,926)	(58,538)
Other Underwriting Income (Expense)		3	21
Underwriting Profit (Loss)		19,407	10,170
Net Investment Income		23,702	21,448
Total Net Realized and Unrealized Losses		(2,012)	(44)
Interest Expense		(3,864)	(3,861)
Other Income (Loss)		(120)	1,047
Income (Loss) Before Income Taxes		37,113	28,760
Income Tax Expense		(6,235)	(7,650)
Net Income (Loss)		$ 30,878	$ 21,110
Losses and LAE Ratio		57.70%	59.30%
Commission Expense Ratio		16.80%	16.70%
Other Operating Expense Ratio	[1]	19.50%	20.40%
Combined Ratio		94.00%	96.40%
U.S.Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 172,913	$ 164,004
Net Losses and LAE		(110,422)	(98,826)
Commission Expenses		(20,861)	(20,384)
Other Operating Expenses		(36,991)	(33,472)
Other Underwriting Income (Expense)		98	110
Underwriting Profit (Loss)		4,737	11,432
Income (Loss) Before Income Taxes		$ 4,737	$ 11,432
Losses and LAE Ratio		63.90%	60.30%
Commission Expense Ratio		12.10%	12.40%
Other Operating Expense Ratio	[1]	21.30%	20.30%
Combined Ratio		97.30%	93.00%
International Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 93,210	$ 84,086
Net Losses and LAE		(45,843)	(50,705)
Commission Expenses		(19,756)	(19,233)
Other Operating Expenses		(20,530)	(19,793)
Underwriting Profit (Loss)		7,081	(5,645)
Income (Loss) Before Income Taxes		$ 7,081	$ (5,645)
Losses and LAE Ratio		49.20%	60.30%
Commission Expense Ratio		21.20%	22.90%
Other Operating Expense Ratio	[1]	22.00%	23.50%
Combined Ratio		92.40%	106.70%
Global Reinsurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 56,504	$ 38,041
Net Losses and LAE		(29,880)	(20,069)
Commission Expenses		(13,768)	(8,492)
Other Operating Expenses		(5,405)	(5,273)
Other Underwriting Income (Expense)		138	176
Underwriting Profit (Loss)		7,589	4,383
Income (Loss) Before Income Taxes		$ 7,589	$ 4,383
Losses and LAE Ratio		52.90%	52.80%
Commission Expense Ratio		24.40%	22.30%
Other Operating Expense Ratio	[1]	9.30%	13.40%
Combined Ratio		86.60%	88.50%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]	$ 23,702	$ 21,448
Total Net Realized and Unrealized Losses	[2]	(2,012)	(44)
Interest Expense	[2]	(3,864)	(3,861)
Other Income (Loss)	[2]	(120)	1,047
Income (Loss) Before Income Taxes	[2]	17,706	18,590
Income Tax Expense	[2]	(6,235)	(7,650)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]	233	265
Other Underwriting Income (Expense)	[2]	$ (233)	$ (265)

[1] Includes Other Operating Expenses and Other Underwriting Income (Expense).

[2] Includes Corporate segment intercompany eliminations.