Notes Payable and Debt and Financing Arrangements - Letter of Credit Summary (Details) (USD $)	Mar. 31, 2013 Credit_Facilities		Dec. 31, 2012 Credit_Facilities
Line of Credit Facility [Line Items]
Number Of Available Letter Of Credit	5		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]
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	4,000,000,000	[1],[3]	4,000,000,000	[1],[3]
Available letter of credit facilities - in use	1,769,005,000	[1]	1,757,250,000	[1]
Collateralized by certain assets of the Company’s investment portfolio	93.40%	[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 March 31, 2013 and December 31, 2012, there were five available letter of credit facilities.

[2] At March 31, 2013 and December 31, 2012, the revolving credit facility was unutilized.

[3] The Company has the option to increase the size of the March 2011 Credit Agreement by an additional $500 million and the size of the facilities under the December 2011 Credit Agreements by an additional $500 million across both such facilities.