Share Capital - Authorized and issued (Details) - USD ($)							12 Months Ended
		Aug. 23, 2017	Jul. 07, 2017	May 01, 2015	Oct. 15, 2011	Mar. 15, 2007	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 17, 2017	May 13, 2016
Shareholders Equity Disclosure [Line Items]
Authorized share capital, value							$ 9,999,900
Ordinary shares, shares authorized (in shares)							999,990,000	999,990,000
Common shares, par value (in dollars per share)							$ 0.01	$ 0.01
Rollforward of ordinary shares issued and outstanding:
Balance at beginning of year, shares							266,889,127	294,745,000	255,183,000
Exercise of options, shares							1,827,000	802,000	480,000
Net issuance of restricted shares, shares							1,138,000	1,598,000	1,581,000
Share buybacks, shares	[1]						(13,820,000)	(30,256,000)	(12,434,000)
Issuance of shares, shares							0	0	49,935,000
Balance at end of year, shares							256,033,895	266,889,127	294,745,000
Ordinary share buybacks:
Shares purchased and canceled during the period							13,800,000
Value of shares purchased and canceled during the period							$ 571,600,000
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period							13,800,000
Articles Of Association maximum voting power, percentage							10.00%
Noncumulative perpetual preferred shares
Ordinary share buybacks:
Shares purchased and canceled during the period			46,635
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period			46,635
Preferred Stock, Value, Outstanding			$ 43,700,000
Payments for Repurchase of Preferred Stock and Preference Stock			45,600,000
Preferred Stock Redemption Premium			$ 1,900,000
Ordinary shares
Shareholders Equity Disclosure [Line Items]
Ordinary shares, shares authorized (in shares)							500,000,000
Common shares, par value (in dollars per share)							$ 0.01
Undesignated shares
Shareholders Equity Disclosure [Line Items]
Ordinary shares, shares authorized (in shares)							499,990,000
Common shares, par value (in dollars per share)							$ 0.01
Series D preference ordinary shares
Ordinary share buybacks:
Shares purchased and canceled during the period		30,652	27,205
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period		30,652	27,205
Proceeds on issuance of preferred shares					$ 350,000,000
Preferred Stock, Value, Outstanding		$ 30,600,000	$ 27,100,000
Payments for Repurchase of Preferred Stock and Preference Stock		$ 29,500,000	$ 25,800,000
Preferred Stock Redemption Discount							$ 2,400,000
Series D preference ordinary shares | LIBOR
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Dividend rate, spread on LIBOR					3.12%
Series E Preferred Stock [Member]
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Dividend rate, spread on LIBOR							6.50%
Series E Preferred Stock [Member] | LIBOR
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Dividend rate, spread on LIBOR							2.4575%
Series E preference ordinary shares
Ordinary share buybacks:
Shares purchased and canceled during the period			329,722
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period			329,722
Liquidation preference per share						$ 1,000.0
Preferred shares, shares issued						1,000,000
Preferred shares, stated dividend per share						$ 32.5
Preferred shares, par value						$ 0.01
Preferred Stock, Value, Outstanding			$ 324,400,000
Payments for Repurchase of Preferred Stock and Preference Stock			310,600,000
Preferred Stock Redemption Discount			$ 13,700,000
Two Thousand Sixteen Share Repurchase Authorization [Member]
Ordinary share buybacks:
Share repurchase authorization											$ 1,000,000,000
Shares purchased and canceled during the period							2,600,000
Value of shares purchased and canceled during the period							$ 100,000,000
Amount remaining under share repurchase plan										$ 349,000,000
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period							2,600,000
Two Thousand Seventeen Share Repurchase Authorization [Member] [Member]
Ordinary share buybacks:
Share repurchase authorization										$ 1,000,000,000
Shares purchased and canceled during the period							11,200,000
Value of shares purchased and canceled during the period							$ 470,900,000
Amount remaining under share repurchase plan							$ 529,100,000
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Shares purchased and canceled during the period							11,200,000
Settling of employee withholding taxes [Member]
Ordinary share buybacks:
Value of shares purchased and canceled during the period							$ 700,000
Catlin Group Limited [Member]
Shareholders Equity Disclosure [Line Items]
Common shares, par value (in dollars per share)				$ 0.01
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
XL Share issuance to Catlin shareholders				49,935,000
XL Share issuance consideration				$ 1,851,601,000
Catlin Group Limited [Member] | Noncumulative perpetual preferred shares
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Liquidation preference per share							$ 1,000
Preferred Stock, Shares Outstanding							600,000
Preferred shares, par value							$ 0.01
Change In Dividend Rate If Shares Not Redeemed at Specified Date [Member] | Catlin Group Limited [Member] | Noncumulative perpetual preferred shares
Preferred shares and non-controlling interest in equity of consolidated subsidiaries:
Dividend rate, spread on LIBOR							2.975%

[1] Includes share buybacks associated with authorized share buyback programs as well as purchases related to satisfying tax withholding obligations of employees in connection with the vesting of restricted shares granted under the Company's equity compensation programs.