Notes Payable and Debt and Financing Arrangements (Details) (USD $)	9 Months Ended										0 Months Ended
	Sep. 30, 2014 Credit_Facilities	Sep. 30, 2014 Revolving credit facility		Dec. 31, 2013 Revolving credit facility		Sep. 30, 2014 Letter of credit facilities		Dec. 31, 2013 Letter of credit facilities		Sep. 30, 2014 November 2013 unsecured credit agreement	Sep. 15, 2014 Senior Notes	Sep. 30, 2014 Two Thousand Thirteen Citi Agreements [Member] Revolving credit facility	Sep. 30, 2014 Two Thousand Thirteen Citi Agreements [Member] Letter of credit facilities	Sep. 30, 2014 Syndicated Credit Agreements [Member]
Line of Credit Facility [Line Items]
Percentage Ownership Of XLIT Ltd.	100.00%
Principal amount repaid at maturity											$ 600,000,000
Debt interest rate (percent)											5.25%
Available letter of credit facilities - commitments		1,000,000,000	[1],[2]	1,000,000,000	[1],[2]	3,575,000,000	[1],[3]	3,575,000,000	[1],[3]			575,000,000
Available letter of credit facilities - in use						1,818,665,000	[1]	1,895,425,000	[1]				575,000,000
Collateralized by certain assets of the Company’s investment portfolio						66.60%	[1]	67.60%	[1]
Number Of Available Letter Of Credit	7
Line Of Credit Facility Optional Increased Capacity										$ 1,000,000,000				$ 500,000,000

[1] At September 30, 2014 and December 31, 2013, there were seven available letter of credit facilities.

[2] At September 30, 2014 and December 31, 2013, the revolving credit available under the November 2013 unsecured credit agreement, which provides for issuance of letters of credit and revolving credit loans up to $1 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. At September 30, 2014, $575.0 million of letters of credit were issued under these agreements and therefore such amount is not included in this line item.

[3] The Company has the option to increase the size of the facilities under the Syndicated Credit 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 Citi Agreements with the lender's and issuing lender's consent.