Statutory Financial Data (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
XL-Ireland
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements	$ 11,000,000		$ 1,600,000
XL-Cayman
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements	1,400,000,000		2,000,000,000
Bermuda
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	817,461,000		614,313,000		1,654,736,000
BSCR level for principal operating subsidiaries	100.00%
Maximum permitted dividends as a percentage of prior year statutory capital and surplus	25.00%
Maximum permitted dividends as a percentage of statutory capital	15.00%
Maximum dividend permitted without affidavit	1,400,000,000		1,300,000,000
Bermuda | Consolidated basis
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	6,555,743,000	[1]	6,500,000,000
Actual statutory capital and surplus	10,343,015,000	[2]
Bermuda | Principal operating subsidiaries
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus			4,923,771,000	[1]
Actual statutory capital and surplus			9,152,303,000	[2]
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	649,166,000	[3]	637,799,000	[3]
Actual statutory capital and surplus	2,237,834,000	[2],[3]	2,093,694,000	[2],[3]
Statutory net income (loss)	106,510,000		23,350,000		116,886,000
RBC level for certain operating subsidiaries	100.00%
Maximum permitted dividends as a percentage of statutory policyholder surplus	10.00%
Maximum permitted dividends as a percentage of "Adjusted net investment income"	100.00%
Maximum dividend permitted without affidavit	152,900,000
Statutory earned surplus, amount	152,900,000		59,700,000
Number of property and casualty subsidiaries	7
Europe, UK, and other
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	1,313,424,000		1,256,865,000
Actual statutory capital and surplus	2,495,982,000	[2]	3,108,457,000	[2]
Statutory net income (loss)	157,012,000		107,255,000		291,223,000
Ireland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	900,000,000
Distributable reserves, amount	3,600,000,000
United Kingdom
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 1,000,000,000

[1] Required statutory capital and surplus at December 31, 2012 represents 100% Bermuda Solvency Capital Requirements ("BSCR") level for the top Bermuda operating subsidiary, XLIB, calculated on a consolidated basis (and therefore includes a BSCR requirement for all regions). Required statutory capital and surplus at December 31, 2011 represents 100% BSCR level for the prinicipal Bermuda operating subsidiaries only. If the December 31, 2011 required statutory capital and surplus had been determined on a consolidated basis, consistent with 2012 requirement, the amount would have been $6.5 billion.

[2] Statutory assets in Bermuda include investments in other U.S. and international subsidiaries reported separately herein.

[3] Required statutory capital and surplus represents 100% RBC level for principal U.S. operating subsidiaries.