Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Gross Written Premiums		$ 455,943	$ 402,038	$ 1,448,403	$ 1,304,522
Ceded written premiums		(103,531)	(106,022)	(323,437)	(338,061)
Net written premiums		$ 352,412	$ 296,016	$ 1,124,966	$ 966,461
Retention Ratio		77.30%	73.60%	77.70%	74.10%
Net Earned Premiums		$ 347,038	$ 301,355	$ 1,000,680	$ 881,321
Net Losses and LAE		(240,900)	(276,171)	(623,378)	(622,881)
Commission Expenses		(57,999)	(45,509)	(165,344)	(141,526)
Other Operating Expenses		(67,584)	(45,773)	(198,692)	(165,077)
Other Underwriting Income (Expense)		953	8	963	40
Underwriting Profit (Loss)		(18,492)	(66,090)	14,229	(48,123)
Net Investment Income		25,651	22,598	73,954	66,311
Total Net Realized and Unrealized Gains		2,563	4,218	3,667	5,868
Interest Expense		(3,891)	(3,862)	(11,619)	(11,584)
Other Income (Loss)		771	(1,707)	3,272	(1,082)
Merger Transaction Costs		(2,439)		(2,439)
Income (Loss) Before Income Taxes		4,163	(44,843)	81,064	11,390
Income Tax Benefit (Expense)		462	16,864	(13,457)	2,243
Net Income (Loss)		$ 4,625	$ (27,979)	$ 67,607	$ 13,633
Losses and LAE Ratio		69.40%	91.60%	62.30%	70.70%
Commission Expense Ratio		16.70%	15.10%	16.50%	16.10%
Other Operating Expense Ratio	[1]	19.20%	15.20%	19.80%	18.70%
Combined Ratio		105.30%	121.90%	98.60%	105.50%
U.S.Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums		$ 280,183	$ 235,052	$ 808,589	$ 729,843
Ceded written premiums		(72,615)	(64,328)	(215,340)	(199,672)
Net written premiums		$ 207,568	$ 170,724	$ 593,249	$ 530,171
Retention Ratio		74.10%	72.60%	73.40%	72.60%
Net Earned Premiums		$ 188,504	$ 169,150	$ 541,163	$ 500,241
Net Losses and LAE		(141,182)	(135,340)	(363,489)	(339,436)
Commission Expenses		(22,247)	(18,286)	(63,490)	(59,130)
Other Operating Expenses		(36,826)	(25,375)	(112,264)	(91,987)
Other Underwriting Income (Expense)		65	125	234	335
Underwriting Profit (Loss)		(11,686)	(9,726)	2,154	10,023
Income (Loss) Before Income Taxes		$ (11,686)	$ (9,726)	$ 2,154	$ 10,023
Losses and LAE Ratio		74.90%	80.00%	67.20%	67.90%
Commission Expense Ratio		11.80%	10.80%	11.70%	11.80%
Other Operating Expense Ratio	[1]	19.50%	14.90%	20.70%	18.30%
Combined Ratio		106.20%	105.70%	99.60%	98.00%
International Insurance
Segment Reporting Information [Line Items]
Gross Written Premiums		$ 117,376	$ 113,601	$ 377,771	$ 386,654
Ceded written premiums		(29,020)	(38,966)	(97,790)	(129,368)
Net written premiums		$ 88,356	$ 74,635	$ 279,981	$ 257,286
Retention Ratio		75.30%	65.70%	74.10%	66.50%
Net Earned Premiums		$ 95,226	$ 84,407	$ 280,507	$ 250,593
Net Losses and LAE		(60,275)	(81,713)	(158,422)	(176,513)
Commission Expenses		(22,547)	(16,201)	(62,166)	(54,435)
Other Operating Expenses		(24,441)	(16,388)	(68,270)	(58,687)
Other Underwriting Income (Expense)		945		945
Underwriting Profit (Loss)		(11,092)	(29,895)	(7,406)	(39,042)
Income (Loss) Before Income Taxes		$ (11,092)	$ (29,895)	$ (7,406)	$ (39,042)
Losses and LAE Ratio		63.30%	96.80%	56.50%	70.40%
Commission Expense Ratio		23.70%	19.20%	22.20%	21.70%
Other Operating Expense Ratio	[1]	24.60%	19.40%	23.90%	23.50%
Combined Ratio		111.60%	135.40%	102.60%	115.60%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross Written Premiums		$ 58,384	$ 53,385	$ 262,043	$ 188,025
Ceded written premiums		(1,896)	(2,728)	(10,307)	(9,021)
Net written premiums		$ 56,488	$ 50,657	$ 251,736	$ 179,004
Retention Ratio		96.80%	94.90%	96.10%	95.20%
Net Earned Premiums		$ 63,308	$ 47,798	$ 179,010	$ 130,487
Net Losses and LAE		(39,443)	(59,118)	(101,467)	(106,932)
Commission Expenses		(13,282)	(11,233)	(40,198)	(28,695)
Other Operating Expenses		(6,317)	(4,010)	(18,158)	(14,403)
Other Underwriting Income (Expense)		20	94	294	439
Underwriting Profit (Loss)		4,286	(26,469)	19,481	(19,104)
Income (Loss) Before Income Taxes		$ 4,286	$ (26,469)	$ 19,481	$ (19,104)
Losses and LAE Ratio		62.30%	123.70%	56.70%	81.90%
Commission Expense Ratio		21.00%	23.50%	22.50%	22.00%
Other Operating Expense Ratio	[1]	9.90%	8.20%	9.90%	10.70%
Combined Ratio		93.20%	155.40%	89.10%	114.60%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]	$ 25,651	$ 22,598	$ 73,954	$ 66,311
Total Net Realized and Unrealized Gains	[2]	2,563	4,218	3,667	5,868
Interest Expense	[2]	(3,891)	(3,862)	(11,619)	(11,584)
Other Income (Loss)	[2]	771	(1,707)	3,272	(1,082)
Merger Transaction Costs	[2]	(2,439)		(2,439)
Income (Loss) Before Income Taxes	[2]	22,655	21,247	66,835	59,513
Income Tax Benefit (Expense)	[2]	462	16,864	(13,457)	2,243
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]	77	211	510	734
Other Underwriting Income (Expense)	[2]	$ (77)	$ (211)	$ (510)	$ (734)

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

[2] Includes Corporate segment intercompany eliminations.