Notes Payable and Debt and Financing Arrangements (Details) (USD $)	9 Months Ended
	Sep. 30, 2013 Credit_Facilities		Dec. 31, 2012 Credit_Facilities
Line of Credit Facility [Line Items]
Percentage Ownership Of XLIT Ltd.	100.00%
Number Of Available Letter Of Credit	7		5
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 1,000,000,000	[1],[2]	$ 1,000,000,000	[1],[2]
Number Of Available Letter Of Credit	2
Line Of Credit Facility Optional Increased Capacity	475,000,000
Letters of Credit Outstanding, Amount	475,000,000
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	4,475,000,000		4,000,000,000	[1],[3]
Available letter of credit facilities - in use	1,859,662,000	[1]	1,757,250,000	[1]
Collateralized by certain assets of the Company’s investment portfolio	74.50%	[1]	93.30%	[1]
March 2011 Credit Agreement
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity	500,000,000
December 2011 Credit Agreement
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity	$ 500,000,000

[1] At September 30, 2013, there were seven available letter of credit facilities; at December 31, 2012, there were five available letter of credit facilities.

[2] At September 30, 2013 and December 31, 2012, the revolving credit available under the December 9, 2011 unsecured credit agreement (the "December 2011 Unsecured Credit Agreement" and, together with the December 9, 2011 secured credit agreement (the "December 2011 Credit Agreements") was unutilized. The two credit agreements with Citicorp USA, Inc., as administrative agent and issuing lender, and the lenders party thereto, each as amended (collectively, the "2013 Credit Agreements"), provide for issuance of letters of credit and revolving credit loans up to an aggregate amount of $475 million. At September 30, 2013, $475 million of letters of credit were issued under the 2013 Credit Agreements and therefore such amount is not included here.

[3] The Company has the option to increase the size of the facility under the March 25, 2011 secured credit agreement, as amended, by an additional $500 million, the size of the facilities under the December 2011 Credit Agreements by an additional $500 million across both such facilities and the maximum amount of the letters of credit and revolving credit loans available under the 2013 Credit Agreements, with the lender's and issuing lender's consent.