Share Capital - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended
	May. 01, 2015	Jun. 30, 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		2,900,000
Stock Repurchased and Retired During Period, Value		$ 110,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 157,600
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 [Member]
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period		40,570
Aggregate Grant Date Fair Value of Award		$ 1,500
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		$ 55,400
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Deferred Compensation, Share-based Payments [Member]
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period		2,500,000
Aggregate Grant Date Fair Value of Award		$ 93,400
Performance Shares [Member]
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		$ 28,200
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Minimum [Member]
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Minimum [Member] | Performance Shares [Member]
Shareholders Equity Disclosure [Line Items]
Performance Units Final Payout Percentage Of Grant Amount		0.00%
Maximum [Member]
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Maximum [Member] | Performance Shares [Member]
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
Preferred Stock, Liquidation Preference Per Share		$ 1,000
Preferred Stock, Dividend Rate, Percentage or Spread on Basis		7.249%
Preferred Stock, Value, Outstanding		$ 562,300
Preferred Stock, Shares Outstanding		600,000
Three Year Option [Member] | Deferred Compensation, Share-based Payments [Member]
Shareholders Equity Disclosure [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Two Year Option [Member] | Deferred Compensation, Share-based Payments [Member]
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%