Financial Data by 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
Retention Ratio		79.50%	79.40%
Net Earned Premiums		$ 377,189	$ 322,627
Net Losses and LAE		(270,352)	(186,145)
Commission Expenses		(67,716)	(54,152)
Other Operating Expenses		(83,756)	(62,926)
Other Underwriting Income (Expense)		1,123	3
Underwriting Loss		(43,512)	19,407
Net Investment Income		28,219	23,702
Total Net Realized and Unrealized Gains		27,162	(2,012)
Interest Expense		(3,867)	(3,864)
Other Income (Loss)		(5,395)	(120)
Merger Transaction Costs		(850)
Income Before Income Taxes		1,757	37,113
Income Tax Expense		(3,495)	(6,235)
Net Income (Loss)		$ (1,738)	$ 30,878
Losses and LAE Ratio		71.70%	57.70%
Commission Expense Ratio		18.00%	16.80%
Other Operating Expense Ratio	[1]	21.80%	19.50%
Combined Ratio		111.50%	94.00%
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
Retention Ratio		74.70%	72.20%
Net Earned Premiums		$ 202,037	$ 172,913
Net Losses and LAE		(151,125)	(110,422)
Commission Expenses		(24,786)	(20,861)
Other Operating Expenses		(43,210)	(36,991)
Other Underwriting Income (Expense)		95	98
Underwriting Loss		(16,989)	4,737
Income Before Income Taxes		$ (16,989)	$ 4,737
Losses and LAE Ratio		74.80%	63.90%
Commission Expense Ratio		12.30%	12.10%
Other Operating Expense Ratio	[1]	21.30%	21.30%
Combined Ratio		108.40%	97.30%
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
Retention Ratio		75.80%	77.40%
Net Earned Premiums		$ 108,913	$ 93,210
Net Losses and LAE		(75,458)	(45,843)
Commission Expenses		(26,447)	(19,756)
Other Operating Expenses		(32,422)	(20,530)
Other Underwriting Income (Expense)		1,034
Underwriting Loss		(24,380)	7,081
Income Before Income Taxes		$ (24,380)	$ 7,081
Losses and LAE Ratio		69.30%	49.20%
Commission Expense Ratio		24.30%	21.20%
Other Operating Expense Ratio	[1]	28.80%	22.00%
Combined Ratio		122.40%	92.40%
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
Retention Ratio		94.10%	94.90%
Net Earned Premiums		$ 66,239	$ 56,504
Net Losses and LAE		(43,769)	(29,880)
Commission Expenses		(16,433)	(13,768)
Other Operating Expenses		(8,124)	(5,405)
Other Underwriting Income (Expense)		(56)	138
Underwriting Loss		(2,143)	7,589
Income Before Income Taxes		$ (2,143)	$ 7,589
Losses and LAE Ratio		66.10%	52.90%
Commission Expense Ratio		24.80%	24.40%
Other Operating Expense Ratio	[1]	12.30%	9.30%
Combined Ratio		103.20%	86.60%
Corporate
Segment Reporting Information [Line Items]
Commission Expenses	[2]	$ (50)	$ 233
Other Underwriting Income (Expense)	[2]	50	(233)
Net Investment Income	[2]	28,219	23,702
Total Net Realized and Unrealized Gains	[2]	27,162	(2,012)
Interest Expense	[2]	(3,867)	(3,864)
Other Income (Loss)	[2]	(5,395)	(120)
Merger Transaction Costs	[2]	(850)
Income Before Income Taxes	[2]	45,269	17,706
Income Tax Expense	[2]	$ (3,495)	$ (6,235)

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

[2] Includes Corporate segment intercompany eliminations.