Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 301,355	$ 284,010	$ 881,321	$ 816,360
Net Losses and LAE		(276,171)	(172,793)	(622,881)	(492,955)
Commission Expenses		(45,509)	(42,611)	(141,526)	(120,891)
Other Operating Expenses		(45,773)	(56,137)	(165,077)	(176,020)
Other Underwriting Income (Expense)		8	44	40	117
Underwriting Profit (Loss)		(66,090)	12,513	(48,123)	26,611
Net Investment Income		22,598	19,875	66,311	59,344
Net Realized Gains		4,218	1,586	5,868	4,993
Interest Expense		(3,862)	(3,859)	(11,584)	(11,575)
Other Income (Loss)		(1,707)	(227)	(1,082)	6,679
Income (Loss) Before Income Taxes		(44,843)	29,888	11,390	86,052
Income Tax Benefit (Expense)		16,864	(7,875)	2,243	(24,917)
Net Income (Loss)		$ (27,979)	$ 22,013	$ 13,633	$ 61,135
Losses and LAE Ratio		91.60%	60.80%	70.70%	60.40%
Commission Expense Ratio		15.10%	15.00%	16.10%	14.80%
Other Operating Expense Ratio	[1]	15.20%	19.80%	18.70%	21.50%
Combined Ratio		121.90%	95.60%	105.50%	96.70%
U.S.Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 169,150	$ 161,722	$ 500,241	$ 462,446
Net Losses and LAE		(135,340)	(106,320)	(339,436)	(291,260)
Commission Expenses		(18,286)	(18,630)	(59,130)	(50,379)
Other Operating Expenses		(25,375)	(31,517)	(91,987)	(96,848)
Other Underwriting Income (Expense)		125	240	335	943
Underwriting Profit (Loss)		(9,726)	5,495	10,023	24,902
Income (Loss) Before Income Taxes		$ (9,726)	$ 5,495	$ 10,023	$ 24,902
Losses and LAE Ratio		80.00%	65.70%	67.90%	63.00%
Commission Expense Ratio		10.80%	11.50%	11.80%	10.90%
Other Operating Expense Ratio	[1]	14.90%	19.40%	18.30%	20.70%
Combined Ratio		105.70%	96.60%	98.00%	94.60%
International Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 84,407	$ 79,761	$ 250,593	$ 235,602
Net Losses and LAE		(81,713)	(45,043)	(176,513)	(133,519)
Commission Expenses		(16,201)	(15,348)	(54,435)	(47,524)
Other Operating Expenses		(16,388)	(20,183)	(58,687)	(64,997)
Underwriting Profit (Loss)		(29,895)	(813)	(39,042)	(10,438)
Income (Loss) Before Income Taxes		$ (29,895)	$ (813)	$ (39,042)	$ (10,438)
Losses and LAE Ratio		96.80%	56.50%	70.40%	56.70%
Commission Expense Ratio		19.20%	19.20%	21.70%	20.20%
Other Operating Expense Ratio	[1]	19.40%	25.30%	23.50%	27.50%
Combined Ratio		135.40%	101.00%	115.60%	104.40%
Global Reinsurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 47,798	$ 42,527	$ 130,487	$ 118,312
Net Losses and LAE		(59,118)	(21,430)	(106,932)	(68,176)
Commission Expenses		(11,233)	(8,951)	(28,695)	(24,188)
Other Operating Expenses		(4,010)	(4,437)	(14,403)	(14,175)
Other Underwriting Income (Expense)		94	122	439	374
Underwriting Profit (Loss)		(26,469)	7,831	(19,104)	12,147
Income (Loss) Before Income Taxes		$ (26,469)	$ 7,831	$ (19,104)	$ 12,147
Losses and LAE Ratio		123.70%	50.40%	81.90%	57.60%
Commission Expense Ratio		23.50%	21.00%	22.00%	20.40%
Other Operating Expense Ratio	[1]	8.20%	10.20%	10.70%	11.70%
Combined Ratio		155.40%	81.60%	114.60%	89.70%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]	$ 22,598	$ 19,875	$ 66,311	$ 59,344
Net Realized Gains	[2]	4,218	1,586	5,868	4,993
Interest Expense	[2]	(3,862)	(3,859)	(11,584)	(11,575)
Other Income (Loss)	[2]	(1,707)	(227)	(1,082)	6,679
Income (Loss) Before Income Taxes	[2]	21,247	17,375	59,513	59,441
Income Tax Benefit (Expense)	[2]	16,864	(7,875)	2,243	(24,917)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]	211	318	734	1,200
Other Underwriting Income (Expense)	[2]	$ (211)	$ (318)	$ (734)	$ (1,200)

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

[2] Includes Corporate segment intercompany eliminations.