Related Party Transactions - Additional Information (Detail) £ in Millions			3 Months Ended									12 Months Ended
	Sep. 29, 2015	Jun. 03, 2015	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jul. 31, 2012 shares	Dec. 31, 2015 USD ($) Company Investment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 GBP (£)	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			$ 3,911,000				$ 217,970,000					$ 3,911,000	$ 217,970,000
Net realized and unrealized gains (losses)			(57,893,000)	$ (15,130,000)	$ (11,249,000)	$ 43,020,000	7,971,000	$ (18,336,000)	$ 38,411,000	$ 34,573,000		(41,252,000)	62,619,000	$ 70,651,000
Reinsurance recoverables			$ 1,474,004,000				1,331,555,000					$ 1,474,004,000	1,331,555,000
CPPIB and Management of Wilton Re
Related Party Transaction [Line Items]
Percentage of common stock			100.00%									100.00%			100.00%
Voting interest held by noncontrolling owners (percent)	13.90%	9.30%
Economic interest held by noncontrolling owners (percent)	13.80%	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%			8.40%
Number of newly elected board of directors | general_partner																1
Number of general partners | general_partner																4
Non-controlling interest			$ 417,700,000				374,600,000					$ 417,700,000	374,600,000
Number of investment of funds affiliated with entities | Investment												4
Fair value of the investments in funds			237,900,000				202,600,000					$ 237,900,000	202,600,000
Net realized and unrealized gains (losses)												(100,000)	(700,000)
Consulting services expense												400,000
Registered Investment Companies
Related Party Transaction [Line Items]
Fair value of the investments in funds			21,000,000				25,600,000					21,000,000	25,600,000
Net realized and unrealized gains (losses)												(4,700,000)	(1,200,000)
Interest income, related party												2,800,000	700,000
Eagle Point
Related Party Transaction [Line Items]
Separate account assets			157,800,000				127,800,000					157,800,000	127,800,000
Management fees												$ 400,000	100,000
Sound Point Capital
Related Party Transaction [Line Items]
Number of investment of funds affiliated with entities | Investment												2
Fair value of the investments in funds			34,500,000				39,900,000					$ 34,500,000	39,900,000
Net realized and unrealized gains (losses)												600,000	800,000
Separate account assets			53,500,000									53,500,000
Management fees												$ 100,000
Goldman Sachs
Related Party Transaction [Line Items]
Number of investment of funds affiliated with entities | Investment												2
Fair value of the investments in funds			39,600,000				36,300,000					$ 39,600,000	36,300,000
Separate account assets			695,800,000									$ 695,800,000
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]
Fair value of the investments in funds			$ 25,100,000									$ 25,100,000
Indirect non-voting investments in number of companies | Company												2
Investment in indirect non-voting interests of affiliated companies | £															£ 44.6
Goldman Sachs Affiliates
Related Party Transaction [Line Items]
Net realized and unrealized gains (losses)												$ 24,100,000	7,000,000
Management fees												$ 600,000
Affiliates interest percentage			38.00%									38.00%			38.00%
Wilton Relife Settlements
Related Party Transaction [Line Items]
Reinsurance recoverables			$ 12,200,000									$ 12,200,000
CLO equities | Sound Point Capital
Related Party Transaction [Line Items]
Fair value of the investments in funds			$ 18,200,000				$ 10,000,000					18,200,000	10,000,000
Net realized and unrealized gains (losses)												(3,600,000)	$ 0
Interest income, related party												$ 3,400,000