Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net earned premiums		$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 251,545	$ 254,082	$ 242,328	$ 236,132	$ 1,100,345	$ 984,087	$ 935,895
Net losses and LAE		(172,493)	(172,793)	(167,206)	(152,956)	(153,881)	(146,546)	(141,973)	(130,198)	(665,448)	(572,598)	(545,229)
Commission expenses		(44,154)	(42,611)	(40,726)	(37,554)	(31,339)	(34,253)	(31,480)	(32,905)	(165,045)	(129,977)	(125,528)
Other operating expenses										(234,096)	(223,516)	(196,825)
Other underwriting income (expense)										136	122	595
Underwriting profit (loss)										35,892	58,118	68,908
Net investment income		20,107	19,875	19,875	19,594	18,499	17,371	16,595	16,253	79,451	68,718	64,168
Net realized gains (losses)										9,036	6,675	12,812
Interest expense		(3,860)	(3,859)	(3,858)	(3,858)	(3,857)	(3,856)	(3,856)	(3,855)	(15,435)	(15,424)	(15,413)
Other income (loss)										8,565	(613)	10,061
Income before income taxes		31,457	29,888	23,243	32,921	23,815	26,924	28,379	38,356	117,509	117,474	140,536
Income tax (expense) benefit		(9,866)	(7,875)	(7,053)	(9,989)	(6,072)	(8,723)	(9,195)	(12,427)	(34,783)	(36,417)	(45,207)
Net income		$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 82,726	$ 81,057	$ 95,329
Losses and LAE ratio										60.50%	58.20%	58.30%
Commission expense ratio										15.00%	13.20%	13.40%
Other operating expense ratio	[1]									21.20%	22.70%	20.90%
Combined ratio		96.70%	95.60%	99.60%	95.10%	97.20%	93.40%	93.40%	92.30%	96.70%	94.10%	92.60%
U.S.Insurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 629,308	$ 555,836	$ 504,289
Net losses and LAE										(397,860)	(343,497)	(311,839)
Commission expenses										(70,812)	(56,319)	(49,840)
Other operating expenses										(128,108)	(131,407)	(115,817)
Other underwriting income (expense)										1,092	1,690	2,241
Underwriting profit (loss)										33,620	26,303	29,034
Income before income taxes										$ 33,620	$ 26,303	$ 29,034
Losses and LAE ratio										63.20%	61.80%	61.80%
Commission expense ratio										11.30%	10.10%	9.90%
Other operating expense ratio	[1]									20.20%	23.40%	22.50%
Combined ratio										94.70%	95.30%	94.20%
International Insurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 307,416	$ 259,960	$ 243,485
Net losses and LAE										(178,284)	(134,702)	(115,079)
Commission expenses										(61,703)	(43,676)	(44,426)
Other operating expenses										(86,395)	(75,867)	(65,275)
Other underwriting income (expense)												32
Underwriting profit (loss)										(18,966)	5,715	18,737
Income before income taxes										$ (18,966)	$ 5,715	$ 18,737
Losses and LAE ratio										58.00%	51.80%	47.30%
Commission expense ratio										20.10%	16.80%	18.20%
Other operating expense ratio	[1]									28.10%	29.20%	26.80%
Combined ratio										106.20%	97.80%	92.30%
Global Reinsurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 163,621	$ 168,291	$ 188,121
Net losses and LAE										(89,304)	(94,399)	(118,311)
Commission expenses										(34,008)	(32,240)	(33,429)
Other operating expenses										(19,593)	(16,242)	(15,733)
Other underwriting income (expense)										522	690	489
Underwriting profit (loss)										21,238	26,100	21,137
Income before income taxes										$ 21,238	$ 26,100	$ 21,137
Losses and LAE ratio										54.60%	56.10%	62.90%
Commission expense ratio										20.80%	19.20%	17.80%
Other operating expense ratio	[1]									11.60%	9.20%	8.10%
Combined ratio										87.00%	84.50%	88.80%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	[2]									$ 1,478	$ 2,258	$ 2,167
Other underwriting income (expense)	[2]									(1,478)	(2,258)	(2,167)
Net investment income	[2]									79,451	68,718	64,168
Net realized gains (losses)	[2]									9,036	6,675	12,812
Interest expense	[2]									(15,435)	(15,424)	(15,413)
Other income (loss)	[2]									8,565	(613)	10,061
Income before income taxes	[2]									81,617	59,356	71,628
Income tax (expense) benefit	[2]									$ (34,783)	$ (36,417)	$ (45,207)

[1] Includes Other operating expenses and Other underwriting income (expense).

[2] Includes Corporate segment intercompany eliminations.