Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net Earned Premiums		$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 1,186,420	$ 1,100,345	$ 984,087
Net Losses and LAE		(183,384)	(276,171)	(177,110)	(169,600)	(172,493)	(172,793)	(167,206)	(152,956)	(806,265)	(665,448)	(572,598)
Commission Expenses		(43,205)	(45,509)	(48,173)	(47,844)	(44,154)	(42,611)	(40,726)	(37,554)	(184,731)	(165,045)	(129,977)
Other Operating Expenses		(68,153)	(45,773)	(60,766)	(58,538)	(58,076)	(56,137)	(59,074)	(60,809)	(233,230)	(234,096)	(223,516)
Other Underwriting Income (Expense)										68	136	122
Underwriting Profit (Loss)										(37,738)	35,892	58,118
Net Investment Income		22,982	22,598	22,265	21,448	20,107	19,875	19,875	19,594	89,293	79,451	68,718
Net Realized Gains										43,009	9,036	6,675
Interest Expense		(3,863)	(3,862)	(3,861)	(3,861)	(3,860)	(3,859)	(3,858)	(3,858)	(15,447)	(15,435)	(15,424)
Other Income (Loss)										(4,311)	8,565	(613)
Income (Loss) Before Income Taxes		63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense		(36,555)	16,864	(6,971)	(7,650)	(9,866)	(7,875)	(7,053)	(9,989)	(34,312)	(34,783)	(36,417)
Net Income (Loss)		$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Losses and LAE ratio										68.00%	60.50%	58.20%
Commission Expense Ratio										15.60%	15.00%	13.20%
Other Operating Expense Ratio	[1]									19.60%	21.20%	22.70%
Combined Ratio		96.60%	121.90%	97.30%	96.40%	96.70%	95.60%	99.60%	95.10%	103.20%	96.70%	94.10%
U.S.Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 674,665	$ 629,308	$ 555,836
Net Losses and LAE										(443,353)	(397,860)	(343,497)
Commission Expenses										(77,729)	(70,812)	(56,319)
Other Operating Expenses										(128,905)	(128,108)	(131,407)
Other Underwriting Income (Expense)										461	1,092	1,690
Underwriting Profit (Loss)										25,139	33,620	26,303
Income (Loss) Before Income Taxes										$ 25,139	$ 33,620	$ 26,303
Losses and LAE ratio										65.70%	63.20%	61.80%
Commission Expense Ratio										11.50%	11.30%	10.10%
Other Operating Expense Ratio	[1]									19.10%	20.20%	23.40%
Combined Ratio										96.30%	94.70%	95.30%
International Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 333,792	$ 307,416	$ 259,960
Net Losses and LAE										(229,601)	(178,284)	(134,702)
Commission Expenses										(68,824)	(61,703)	(43,676)
Other Operating Expenses										(83,464)	(86,395)	(75,867)
Underwriting Profit (Loss)										(48,097)	(18,966)	5,715
Income (Loss) Before Income Taxes										$ (48,097)	$ (18,966)	$ 5,715
Losses and LAE ratio										68.80%	58.00%	51.80%
Commission Expense Ratio										20.60%	20.10%	16.80%
Other Operating Expense Ratio	[1]									25.00%	28.10%	29.20%
Combined Ratio										114.40%	106.20%	97.80%
Global Reinsurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 177,963	$ 163,621	$ 168,291
Net Losses and LAE										(133,311)	(89,304)	(94,399)
Commission Expenses										(39,136)	(34,008)	(32,240)
Other Operating Expenses										(20,861)	(19,593)	(16,242)
Other Underwriting Income (Expense)										565	522	690
Underwriting Profit (Loss)										(14,780)	21,238	26,100
Income (Loss) Before Income Taxes										$ (14,780)	$ 21,238	$ 26,100
Losses and LAE ratio										74.90%	54.60%	56.10%
Commission Expense Ratio										22.00%	20.80%	19.20%
Other Operating Expense Ratio	[1]									11.40%	11.60%	9.20%
Combined Ratio										108.30%	87.00%	84.50%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]									$ 89,293	$ 79,451	$ 68,718
Net Realized Gains	[2]									43,009	9,036	6,675
Interest Expense	[2]									(15,447)	(15,435)	(15,424)
Other Income (Loss)	[2]									(4,311)	8,565	(613)
Income (Loss) Before Income Taxes	[2]									112,544	81,617	59,356
Income Tax Expense	[2]									(34,312)	(34,783)	(36,417)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]									958	1,478	2,258
Other Underwriting Income (Expense)	[2]									$ (958)	$ (1,478)	$ (2,258)

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

[2] Includes Corporate segment intercompany eliminations.