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	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	$ 319,178	$ 348,011
Gross Unrealized Losses, 12 Months or Greater	(15,381)	(6,333)
Fair Value, Less Than 12 Months	509,172	85,045
Gross Unrealized Losses, Less Than 12 Months	(17,574)	(1,979)
Fair Value, Total	828,350	433,056
Gross Unrealized Losses, Total	(32,955)	(8,312)
Fixed maturity and short-term investments, at fair value
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	78,533	70,831
Gross Unrealized Losses, 12 Months or Greater	(5,040)	(2,688)
Fair Value, Less Than 12 Months	157,430	78,770
Gross Unrealized Losses, Less Than 12 Months	(2,666)	(1,934)
Fair Value, Total	235,963	149,601
Gross Unrealized Losses, Total	(7,706)	(4,622)
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	528
Gross Unrealized Losses, 12 Months or Greater	(2)	(1)
Fair Value, Less Than 12 Months	21,694	3,678
Gross Unrealized Losses, Less Than 12 Months	(339)	(6)
Fair Value, Total	22,217	4,206
Gross Unrealized Losses, Total	(341)	(7)
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	18,995	17,051
Gross Unrealized Losses, 12 Months or Greater	(2,633)	(1,534)
Fair Value, Less Than 12 Months	50,080	20,300
Gross Unrealized Losses, Less Than 12 Months	(1,256)	(1,271)
Fair Value, Total	69,075	37,351
Gross Unrealized Losses, Total	(3,889)	(2,805)
Fixed maturity and short-term investments, at fair value | Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	54,295	39,964
Gross Unrealized Losses, 12 Months or Greater	(2,394)	(1,003)
Fair Value, Less Than 12 Months	81,047	40,072
Gross Unrealized Losses, Less Than 12 Months	(1,035)	(650)
Fair Value, Total	135,342	80,036
Gross Unrealized Losses, Total	(3,429)	(1,653)
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	2,073
Gross Unrealized Losses, 12 Months or Greater	(1)	(138)
Fair Value, Less Than 12 Months	0	0
Gross Unrealized Losses, Less Than 12 Months	0	0
Fair Value, Total	71	2,073
Gross Unrealized Losses, Total	(1)	(138)
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	11,215
Gross Unrealized Losses, 12 Months or Greater	(10)	(12)
Fair Value, Less Than 12 Months	0	14,720
Gross Unrealized Losses, Less Than 12 Months	0	(7)
Fair Value, Total	4,649	25,935
Gross Unrealized Losses, Total	(10)	(19)
Fixed maturity investments, at amortized cost
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	240,645	277,180
Gross Unrealized Losses, 12 Months or Greater	(10,341)	(3,645)
Fair Value, Less Than 12 Months	351,742	6,275
Gross Unrealized Losses, Less Than 12 Months	(14,908)	(45)
Fair Value, Total	592,387	283,455
Gross Unrealized Losses, Total	(25,249)	(3,690)
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	7,221	7,312
Gross Unrealized Losses, 12 Months or Greater	(48)	(19)
Fair Value, Less Than 12 Months	12,024	245
Gross Unrealized Losses, Less Than 12 Months	(410)	(1)
Fair Value, Total	19,245	7,557
Gross Unrealized Losses, Total	(458)	(20)
Fixed maturity investments, at amortized cost | Non-U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	24,424	25,960
Gross Unrealized Losses, 12 Months or Greater	(1,255)	(249)
Fair Value, Less Than 12 Months	8,885	0
Gross Unrealized Losses, Less Than 12 Months	(238)	0
Fair Value, Total	33,309	25,960
Gross Unrealized Losses, Total	(1,493)	(249)
Fixed maturity investments, at amortized cost | Corporate
Schedule of Available-for-sale Securities [Line Items]
Fair Value, 12 Months or Greater	209,000	243,908
Gross Unrealized Losses, 12 Months or Greater	(9,038)	(3,377)
Fair Value, Less Than 12 Months	330,833	6,030
Gross Unrealized Losses, Less Than 12 Months	(14,260)	(44)
Fair Value, Total	539,833	249,938
Gross Unrealized Losses, Total	$ (23,298)	$ (3,421)