Related Party Transactions - Additional Information (Detail) £ in Millions		3 Months Ended
	Jun. 03, 2015	Mar. 31, 2016 USD ($) Company	Mar. 31, 2015 USD ($)	Jul. 31, 2012 shares	Mar. 31, 2016 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)	Sep. 29, 2015	Nov. 06, 2013 general_partner
Related Party Transaction [Line Items]
Shareholders' agreement period		90 days
Redeemable noncontrolling interest, redemption period one (years)		5 years
Redeemable noncontrolling interest, redemption period two (years)		7 years
Non-controlling interest		$ 3,890,000				$ 3,911,000
Net realized and unrealized gains (losses)		37,964,000	$ 43,020,000
Reinsurance recoverables		$ 1,402,216,000				1,474,004,000
CPPIB and Management of Wilton Re
Related Party Transaction [Line Items]
Percentage of common stock		100.00%			100.00%
Voting interest held by noncontrolling owners (percent)	9.30%							4.60%
Economic interest held by noncontrolling owners (percent)	9.90%
Trident
Related Party Transaction [Line Items]
Related party transaction, shares acquired | shares				1,350,000
Percentage of Company's voting ordinary shares acquired		8.40%			8.40%
Number of newly elected board of directors | general_partner									1
Number of general partners | general_partner									4
Non-controlling interest		$ 429,100,000				417,700,000
Fair value of the investments in funds		184,700,000				237,900,000
Net realized and unrealized gains (losses)		200,000	2,300,000
Registered Investment Companies
Related Party Transaction [Line Items]
Fair value of the investments in funds		20,900,000				21,000,000
Net realized and unrealized gains (losses)		(100,000)	1,100,000
Interest income, related party		500,000	800,000
Eagle Point
Related Party Transaction [Line Items]
Separate account assets		157,000,000				157,800,000
Management fees		100,000	100,000
Sound Point Capital
Related Party Transaction [Line Items]
Fair value of the investments in funds		23,100,000				34,500,000
Net realized and unrealized gains (losses)		(400,000)	1,300,000
Separate account assets		54,500,000				53,500,000
Management fees		100,000	0
Goldman Sachs
Related Party Transaction [Line Items]
Fair value of the investments in funds		39,600,000				39,600,000
Separate account assets		$ 775,000,000				758,900,000
Ordinary shares (percent)		4.10%
Goldman Sachs | Series C Non-Voting Ordinary Shares
Related Party Transaction [Line Items]
Ordinary shares (percent)		100.00%
Hastings Insurance Group Limited
Related Party Transaction [Line Items]
Indirect non-voting investments in number of companies | Company		2
Investment in indirect non-voting interests of affiliated companies | £					£ 46.3		£ 44.6
Goldman Sachs Affiliates
Related Party Transaction [Line Items]
Net realized and unrealized gains (losses)		$ 2,500,000	(1,700,000)
Management fees		$ 200,000	100,000
Affiliates interest percentage		38.00%			38.00%
Wilton Relife Settlements
Related Party Transaction [Line Items]
Reinsurance recoverables		$ 12,200,000
CLO equities | Sound Point Capital
Related Party Transaction [Line Items]
Fair value of the investments in funds		17,300,000				$ 18,200,000
Net realized and unrealized gains (losses)		(900,000)	$ 0
Interest income, related party		$ 2,100,000