Notes Payable and Debt and Financing Arrangements (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015 Credit_Facilities		Mar. 30, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Percentage Ownership Of XLIT Ltd.	100.00%
Number Of Available Letter Of Credit	8
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]
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	3,575,000,000	[1],[3]		3,575,000,000	[1],[3]
Available letter of credit facilities - in use	1,743,230,000	[1]		1,790,561,000	[1]
Collateralized by certain assets of the Company’s investment portfolio	65.70%	[1]		66.20%	[1]
November 2013 unsecured credit agreement
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity	2,000,000,000
Two Thousand Thirteen Citi Agreements [Member] | Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	575,000,000
Two Thousand Thirteen Citi Agreements [Member] | Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	575,000,000
Syndicated Credit Agreements [Member]
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity	500,000,000
XL Cayman | 4.45% subordinated notes due 2025 | Subordinated notes
Line of Credit Facility [Line Items]
Face value of issued notes			500,000,000
Debt interest rate (percent)			4.45%
Debt issue price, as a percentage of face value			99.633%
Net proceeds on issuance of debt			492,200,000
Debt issuance cost			5,900,000
XL Cayman | 5.5% subordinated notes due 2045 | Subordinated notes
Line of Credit Facility [Line Items]
Face value of issued notes			500,000,000
Debt interest rate (percent)			5.50%
Debt issue price, as a percentage of face value			99.115%
Net proceeds on issuance of debt			488,400,000
Debt issuance cost			$ 7,200,000

[1] As of March 31, 2015 and December 31, 2014, there were eight available letter of credit facilities.

[2] As of March 31, 2015 and December 31, 2014, the $1 billion of revolving credit available under the November 2013 unsecured credit agreements, which currently provide an aggregate amount for issuance of letters of credit and revolving credit loans up to $2 billion, was unutilized. The credit agreements entered into with Citicorp USA, Inc. in May through November 2013 (the "Citi Agreements") provide for issuance of letters of credit and revolving credit loans up to an aggregate amount of $575.0 million. As of March 31, 2015, $575.0 million of letters of credit were issued under these agreements and therefore such amount is not included in this line.

[3] ovember 2013 credit agreements, which currently provide an aggregate amount of outstanding letters of credit and revolving credit loans of up to $2 billion. The Company has the option to increase the size of the facilities under these agreements by an additional $500 million across such facilities. The Company also has the option to increase the maximum amount of the letters of credit and revolving credit loans available under the 2013 credit agreements with Citicorp USA, Inc. with the lender's and issuing lender's consent.