Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 331,015	$ 293,835	$ 653,642	$ 579,966
Net Losses and LAE		(196,333)	(177,110)	(382,478)	(346,710)
Commission Expenses		(53,193)	(48,173)	(107,345)	(96,017)
Other Operating Expenses		(68,182)	(60,766)	(131,108)	(119,304)
Other Underwriting Income (Expense)		7	11	10	32
Underwriting Profit (Loss)		13,314	7,797	32,721	17,967
Net Investment Income		24,601	22,265	48,303	43,713
Total Net Realized and Unrealized Gains		3,116	1,694	1,104	1,650
Interest Expense		(3,864)	(3,861)	(7,728)	(7,722)
Other Income (Loss)		2,621	(422)	2,501	625
Income (Loss) Before Income Taxes		39,788	27,473	76,901	56,233
Income Tax Expense		(7,684)	(6,971)	(13,919)	(14,621)
Net Income (Loss)		$ 32,104	$ 20,502	$ 62,982	$ 41,612
Losses and LAE Ratio		59.30%	60.30%	58.50%	59.80%
Commission Expense Ratio		16.10%	16.40%	16.40%	16.60%
Other Operating Expense Ratio	[1]	20.60%	20.60%	20.10%	20.50%
Combined Ratio		96.00%	97.30%	95.00%	96.90%
U.S.Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 179,746	$ 167,087	$ 352,659	$ 331,091
Net Losses and LAE		(111,885)	(105,270)	(222,307)	(204,096)
Commission Expenses		(20,382)	(20,460)	(41,243)	(40,844)
Other Operating Expenses		(38,447)	(33,140)	(75,438)	(66,612)
Other Underwriting Income (Expense)		71	100	169	210
Underwriting Profit (Loss)		9,103	8,317	13,840	19,749
Income (Loss) Before Income Taxes		$ 9,103	$ 8,317	$ 13,840	$ 19,749
Losses and LAE Ratio		62.20%	63.00%	63.00%	61.60%
Commission Expense Ratio		11.30%	12.20%	11.70%	12.30%
Other Operating Expense Ratio	[1]	21.40%	19.80%	21.40%	20.10%
Combined Ratio		94.90%	95.00%	96.10%	94.00%
International Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 92,071	$ 82,100	$ 185,281	$ 166,186
Net Losses and LAE		(52,304)	(44,095)	(98,147)	(94,800)
Commission Expenses		(19,863)	(19,001)	(39,619)	(38,234)
Other Operating Expenses		(23,299)	(22,506)	(43,829)	(42,299)
Underwriting Profit (Loss)		(3,395)	(3,502)	3,686	(9,147)
Income (Loss) Before Income Taxes		$ (3,395)	$ (3,502)	$ 3,686	$ (9,147)
Losses and LAE Ratio		56.80%	53.70%	53.00%	57.00%
Commission Expense Ratio		21.60%	23.10%	21.40%	23.00%
Other Operating Expense Ratio	[1]	25.30%	27.50%	23.60%	25.50%
Combined Ratio		103.70%	104.30%	98.00%	105.50%
Global Reinsurance
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 59,198	$ 44,648	$ 115,702	$ 82,689
Net Losses and LAE		(32,144)	(27,745)	(62,024)	(47,814)
Commission Expenses		(13,148)	(8,970)	(26,916)	(17,462)
Other Operating Expenses		(6,436)	(5,120)	(11,841)	(10,393)
Other Underwriting Income (Expense)		136	169	274	345
Underwriting Profit (Loss)		7,606	2,982	15,195	7,365
Income (Loss) Before Income Taxes		$ 7,606	$ 2,982	$ 15,195	$ 7,365
Losses and LAE Ratio		54.30%	62.10%	53.60%	57.80%
Commission Expense Ratio		22.20%	20.10%	23.30%	21.10%
Other Operating Expense Ratio	[1]	10.70%	11.10%	10.00%	12.20%
Combined Ratio		87.20%	93.30%	86.90%	91.10%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]	$ 24,601	$ 22,265	$ 48,303	$ 43,713
Total Net Realized and Unrealized Gains	[2]	3,116	1,694	1,104	1,650
Interest Expense	[2]	(3,864)	(3,861)	(7,728)	(7,722)
Other Income (Loss)	[2]	2,621	(422)	2,501	625
Income (Loss) Before Income Taxes	[2]	26,474	19,676	44,179	38,266
Income Tax Expense	[2]	(7,684)	(6,971)	(13,919)	(14,621)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]	200	258	433	523
Other Underwriting Income (Expense)	[2]	$ (200)	$ (258)	$ (433)	$ (523)

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

[2] Includes Corporate segment intercompany eliminations.