Investments - Summary of Investments Classified as Available-for-Sale in Unrealized Loss Position as Well as Aggregate Fair Value and Gross Unrealized Loss by Length of Time (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	$ 83,121	$ 319,178
Gross Unrealized Losses, 12 Months or Greater	(6,307)	(15,381)
Fair Value, Less Than 12 Months	187,656	509,172
Gross Unrealized Losses, Less Than 12 Months	(6,137)	(17,574)
Fair Value, Total	270,777	828,350
Gross Unrealized Losses, Total	(12,444)	(32,955)
Fixed maturity and short-term investments, at fair value
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	43,152	78,533
Gross Unrealized Losses, 12 Months or Greater	(3,732)	(5,040)
Fair Value, Less Than 12 Months	31,564	157,430
Gross Unrealized Losses, Less Than 12 Months	(494)	(2,666)
Fair Value, Total	74,716	235,963
Gross Unrealized Losses, Total	(4,226)	(7,706)
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater		523
Gross Unrealized Losses, 12 Months or Greater		(2)
Fair Value, Less Than 12 Months		21,694
Gross Unrealized Losses, Less Than 12 Months		(339)
Fair Value, Total		22,217
Gross Unrealized Losses, Total		(341)
Fixed maturity and short-term investments, at fair value | Non-U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	14,821	18,995
Gross Unrealized Losses, 12 Months or Greater	(1,995)	(2,633)
Fair Value, Less Than 12 Months	10,222	50,080
Gross Unrealized Losses, Less Than 12 Months	(281)	(1,256)
Fair Value, Total	25,043	69,075
Gross Unrealized Losses, Total	(2,276)	(3,889)
Fixed maturity and short-term investments, at fair value | Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	28,331	54,295
Gross Unrealized Losses, 12 Months or Greater	(1,737)	(2,394)
Fair Value, Less Than 12 Months	21,342	81,047
Gross Unrealized Losses, Less Than 12 Months	(213)	(1,035)
Fair Value, Total	49,673	135,342
Gross Unrealized Losses, Total	(1,950)	(3,429)
Fixed maturity and short-term investments, at fair value | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater		0
Gross Unrealized Losses, 12 Months or Greater		0
Fair Value, Less Than 12 Months		4,609
Gross Unrealized Losses, Less Than 12 Months		(36)
Fair Value, Total		4,609
Gross Unrealized Losses, Total		(36)
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater		71
Gross Unrealized Losses, 12 Months or Greater		(1)
Fair Value, Less Than 12 Months		0
Gross Unrealized Losses, Less Than 12 Months		0
Fair Value, Total		71
Gross Unrealized Losses, Total		(1)
Fixed maturity and short-term investments, at fair value | Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater		4,649
Gross Unrealized Losses, 12 Months or Greater		(10)
Fair Value, Less Than 12 Months		0
Gross Unrealized Losses, Less Than 12 Months		0
Fair Value, Total		4,649
Gross Unrealized Losses, Total		(10)
Fixed maturity investments, at amortized cost
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	39,969	240,645
Gross Unrealized Losses, 12 Months or Greater	(2,575)	(10,341)
Fair Value, Less Than 12 Months	156,092	351,742
Gross Unrealized Losses, Less Than 12 Months	(5,643)	(14,908)
Fair Value, Total	196,061	592,387
Gross Unrealized Losses, Total	(8,218)	(25,249)
Fixed maturity investments, at amortized cost | U.S. Government and Agency
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	0	7,221
Gross Unrealized Losses, 12 Months or Greater	0	(48)
Fair Value, Less Than 12 Months	458	12,024
Gross Unrealized Losses, Less Than 12 Months	(50)	(410)
Fair Value, Total	458	19,245
Gross Unrealized Losses, Total	(50)	(458)
Fixed maturity investments, at amortized cost | Non-U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	6,323	24,424
Gross Unrealized Losses, 12 Months or Greater	(248)	(1,255)
Fair Value, Less Than 12 Months	14,706	8,885
Gross Unrealized Losses, Less Than 12 Months	(326)	(238)
Fair Value, Total	21,029	33,309
Gross Unrealized Losses, Total	(574)	(1,493)
Fixed maturity investments, at amortized cost | Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	33,646	209,000
Gross Unrealized Losses, 12 Months or Greater	(2,327)	(9,038)
Fair Value, Less Than 12 Months	140,928	330,833
Gross Unrealized Losses, Less Than 12 Months	(5,267)	(14,260)
Fair Value, Total	174,574	539,833
Gross Unrealized Losses, Total	$ (7,594)	$ (23,298)