Notes Payable and Debt and Financing Arrangements (Details) - Narrative relating to long term debt (USD $)	1 Months Ended		12 Months Ended			3 Months Ended
	Aug. 31, 2011	Aug. 31, 2008	Dec. 31, 2012	Dec. 31, 2011	Aug. 31, 2011 8.25% senior notes	Sep. 30, 2011 5.75% Senior Notes due 2021	Dec. 31, 2012 Senior notes 6.5% Guaranteed Senior Notes due 2012	Dec. 31, 2011 Senior notes 6.5% Guaranteed Senior Notes due 2012	Dec. 31, 2012 Senior notes 5.75% Senior Notes due 2021	Dec. 31, 2011 Senior notes 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Percentage ownership of XLIT Ltd. and XL Capital Finance (Europe) plc			100.00%
Number of equity units issued		23,000,000
Rate of interest of equity security units	10.75%
Proceeds on issuance of equity security units		$ 557,000,000
Principal amount of senior notes, per equity unit		$ 1,000
Amount charged to additional paid in capital on issuance of equity security units	37,900,000
Costs accrued on issuance of equity security units	18,000,000
Portion of accrued costs charged to additional paid in capital on issuance of equity security units	14,700,000
Face value of senior notes acquired and retired					575,000,000
Debt instrument face amount								600,000,000	400,000,000	400,000,000
Debt instrument interest rate, stated percentage					8.25%		6.50%		5.75%
Coupon on 10.75% units		2.50%
Equity security unit exchange ratio, in shares	1.3242
Price per share on forward contract	$ 25.00	$ 25.00
Issue of shares, shares	30,456,600		0	30,457,000
Proceeds on issuance of long term debt						$ 395,700,000