Share Capital - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended
	May. 01, 2015	Sep. 30, 2015	Sep. 30, 2015	Aug. 06, 2015	Feb. 21, 2014
2014 Repurchase Authorization
Shareholders Equity Disclosure [Line Items]
Stock Repurchase Program, Authorized Amount					$ 1,000,000
Stock Repurchased and Retired During Period, Shares		1,600,000	4,500,000
Stock Repurchased and Retired During Period, Value		$ 60,000	$ 170,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 97,600
Two Thousand Fifteen Share Repurchase Authorization
Shareholders Equity Disclosure [Line Items]
Stock Repurchase Program, Authorized Amount				$ 1,000,000
Stock Repurchased and Retired During Period, Shares		3,200,000
Stock Repurchased and Retired During Period, Value		$ 120,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 880,000	$ 880,000
Stock options
Shareholders Equity Disclosure [Line Items]
Weighted Average Grant Date Fair Value Per Stock Option			$ 6.51
Number of Stock Options Granted During the Period			2,000,000
Restricted Stock
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period			43,457
Aggregate Grant Date Fair Value of Award			$ 1,600
Restricted stock units
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period			1,500,000
Aggregate Grant Date Fair Value of Award			$ 55,500
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Deferred Compensation, Share-based Payments
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period			2,600,000
Aggregate Grant Date Fair Value of Award			$ 94,500
Performance Shares
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period			800,000
Share Based Compensation Arrangement Performance Units Maximum Share Payout Number			1,600,000
Aggregate Grant Date Fair Value of Award			$ 28,100
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Minimum
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Minimum | Performance Shares
Shareholders Equity Disclosure [Line Items]
Performance Units Final Payout Percentage Of Grant Amount			0.00%
Maximum
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			4 years
Maximum | Performance Shares
Shareholders Equity Disclosure [Line Items]
Performance Units Final Payout Percentage Of Grant Amount			200.00%
Catlin Group Limited
Shareholders Equity Disclosure [Line Items]
Share issuance to Catlin shareholders	49,935,000
Share issuance consideration	$ 1,851,601
Preferred Stock, Par or Stated Value Per Share		$ 0.01	$ 0.01
Preferred Stock, Liquidation Preference Per Share		$ 1,000	$ 1,000
Preferred Stock, Dividend Rate, Percentage or Spread on Basis			7.249%
Preferred Stock, Value, Outstanding		$ 562,300	$ 562,300
Preferred Stock, Shares Outstanding		600,000	600,000
Three Year Option | Deferred Compensation, Share-based Payments
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Two Year Option | Deferred Compensation, Share-based Payments
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			2 years
3 month LIBOR | Catlin Group Limited
Shareholders Equity Disclosure [Line Items]
Preferred Stock, Dividend Rate, Percentage or Spread on Basis			2.975%