Financial Data by Segment (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net earned premiums		$ 251,545	$ 254,082	$ 242,328	$ 236,132	$ 234,172	$ 245,367	$ 231,084	$ 225,272	$ 984,087	$ 935,895	$ 841,939
Net losses and LAE		(153,881)	(146,546)	(141,973)	(130,198)	(134,658)	(135,284)	(140,220)	(135,067)	(572,598)	(545,229)	(518,961)
Commission expenses		(31,339)	(34,253)	(31,480)	(32,905)	(33,708)	(33,943)	(32,150)	(25,727)	(129,977)	(125,528)	(113,494)
Other operating expenses										(223,516)	(196,825)	(164,434)
Other underwriting income (expense)										122	595	(1,172)
Underwriting profit (loss)										58,118	68,908	43,878
Net investment income		18,499	17,371	16,595	16,253	16,071	15,839	15,648	16,610	68,718	64,168	56,251
Net realized gains (losses)										6,675	12,812	20,546
Interest expense		(3,857)	(3,856)	(3,856)	(3,855)	(3,854)	(3,388)	(4,319)	(3,852)	(15,424)	(15,413)	(10,507)
Call premium on Senior notes												(17,895)
Other income (loss)										(613)	10,061
Income before income taxes		23,815	26,924	28,379	38,356	28,098	46,257	24,859	41,322	117,474	140,536	92,273
Income tax (expense) benefit		(6,072)	(8,723)	(9,195)	(12,427)	(8,823)	(15,032)	(7,998)	(13,354)	(36,417)	(45,207)	(28,807)
Net income		$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 19,275	$ 31,225	$ 16,861	$ 27,968	$ 81,057	$ 95,329	$ 63,466
Losses and LAE ratio										58.20%	58.30%	61.60%
Commission expense ratio										13.20%	13.40%	13.50%
Other operating expense ratio	[1]									22.70%	20.90%	19.70%
Combined ratio		97.20%	93.40%	93.40%	92.30%	93.80%	89.50%	95.30%	92.20%	94.10%	92.60%	94.80%
U.S.Insurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 555,836	$ 504,289	$ 437,737
Net losses and LAE										(343,497)	(311,839)	(283,694)
Commission expenses										(56,319)	(49,840)	(43,409)
Other operating expenses										(131,407)	(115,817)	(99,013)
Other underwriting income (expense)										1,690	2,241	2,924
Underwriting profit (loss)										26,303	29,034	14,545
Income before income taxes										$ 26,303	$ 29,034	$ 14,545
Losses and LAE ratio										61.80%	61.80%	64.80%
Commission expense ratio										10.10%	9.90%	9.90%
Other operating expense ratio	[1]									23.40%	22.50%	22.00%
Combined ratio										95.30%	94.20%	96.70%
International Insurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 259,960	$ 243,485	$ 232,962
Net losses and LAE										(134,702)	(115,079)	(103,853)
Commission expenses										(43,676)	(44,426)	(42,977)
Other operating expenses										(75,867)	(65,275)	(54,062)
Other underwriting income (expense)											32	(1,867)
Underwriting profit (loss)										5,715	18,737	30,203
Income before income taxes										$ 5,715	$ 18,737	$ 30,203
Losses and LAE ratio										51.80%	47.30%	44.60%
Commission expense ratio										16.80%	18.20%	18.40%
Other operating expense ratio	[1]									29.20%	26.80%	24.00%
Combined ratio										97.80%	92.30%	87.00%
Global Reinsurance
Segment Reporting Information [Line Items]
Net earned premiums										$ 168,291	$ 188,121	$ 171,240
Net losses and LAE										(94,399)	(118,311)	(131,414)
Commission expenses										(32,240)	(33,429)	(29,456)
Other operating expenses										(16,242)	(15,733)	(11,359)
Other underwriting income (expense)										690	489	119
Underwriting profit (loss)										26,100	21,137	(870)
Income before income taxes										$ 26,100	$ 21,137	$ (870)
Losses and LAE ratio										56.10%	62.90%	76.70%
Commission expense ratio										19.20%	17.80%	17.20%
Other operating expense ratio	[1]									9.20%	8.10%	6.60%
Combined ratio										84.50%	88.80%	100.50%
Corporate
Segment Reporting Information [Line Items]
Commission expenses	[2]									$ 2,258	$ 2,167	$ 2,348
Other underwriting income (expense)	[2]									(2,258)	(2,167)	(2,348)
Net investment income	[2]									68,718	64,168	56,251
Net realized gains (losses)	[2]									6,675	12,812	20,546
Interest expense	[2]									(15,424)	(15,413)	(10,507)
Call premium on Senior notes	[2]											(17,895)
Other income (loss)	[2]									(613)	10,061
Income before income taxes	[2]									59,356	71,628	48,395
Income tax (expense) benefit	[2]									$ (36,417)	$ (45,207)	$ (28,807)

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

[2] Includes Corporate segment intercompany eliminations.