Notes Payable and Debt and Financing Arrangements - Letter of Credit Summary (Details) (USD $)	Jun. 30, 2013 Credit_Facilities		Dec. 31, 2012 Credit_Facilities
Line of Credit Facility [Line Items]
Number Of Available Letter Of Credit	6		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]
Line Of Credit Facility Optional Increased Capacity	275,000,000
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	4,275,000,000	[1],[3]	4,000,000,000	[1],[3]
Available letter of credit facilities - in use	1,891,969,000	[1]	1,757,250,000	[1]
Collateralized by certain assets of the Company’s investment portfolio	85.50%	[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 June 30, 2013, there were six available letter of credit facilities; at December 31, 2012, there were five available letter of credit facilities.

[2] At June 30, 2013 and December 31, 2012, the revolving credit available under the December 2011 Credit Agreements (defined below) was unutilized. The credit agreement with Citicorp USA, Inc., as administrative agent and issuing lender, and the lenders party thereto, as amended (the "2013 Credit Agreement"), provides for issuance of letters of credit and revolving credit loans up to $275 million. At June 30, 2013, $275 million of letters of credit were issued under the 2013 Credit Agreement and therefore such amount is not included here.

[3] The Company has the option to increase the size of the March 2011 Credit Agreement 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 Agreement, with the lender's and issuing lender's consent.