Notes Payable and Debt and Financing Arrangements - Narrative relating to credit facilities (Details) (USD $)	1 Months Ended	0 Months Ended
	Nov. 30, 2013 Credit_Facilities	Sep. 15, 2014	Dec. 31, 2014 Credit_Facilities		Dec. 31, 2013 Credit_Facilities
Line of Credit Facility [Line Items]
Number of letter of credit facilities available			8		7
Syndicated Credit Agreements
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 2,000,000,000
Optional increase capacity			500,000,000
Number of credit facilities entered into	2
2013 secured credit agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments			1,000,000,000
2013 unsecured credit agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments			1,000,000,000
2013 Citi Agreements
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments			575,000,000
2011 credit agreements
Line of Credit Facility [Line Items]
Terminated credit capacity	3,000,000,000
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use			0	[1]	0	[1]
Available letter of credit facilities - commitments			1,000,000,000	[2],[3]	1,000,000,000	[2],[3]
Revolving credit facility | 2013 Citi Agreements
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments			575,000,000
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use			1,790,561,000	[1],[2]	1,895,425,000	[1],[2]
Available letter of credit facilities - commitments			3,575,000,000	[2],[4]	3,575,000,000	[2],[4]
Letter of credit facility | 2013 Citi Agreements
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use			575,000,000
Senior notes
Line of Credit Facility [Line Items]
Extinguishment of Debt, Amount		$ 600,000,000
Debt instrument interest rate, stated percentage		5.25%

[1] “In Use” and “Outstanding” data represent December 31, 2014 and 2013 accreted values.

[2] At December 31, 2014, there were eight available letter of credit facilities; at December 31, 2013, there were seven available letter of credit facilities.

[3] At December 31, 2014 and 2013, the revolving credit available under the Syndicated Credit Agreements (as defined below) and under the December 9, 2011 unsecured credit agreement, respectively, was unutilized. The 2013 Citi Agreements (as defined below) provide for issuance of letters of credit and revolving credit loans in an aggregate amount of up to $575 million. At December 31, 2014, $575 million of letters of credit were issued under the 2013 Citi Agreements and therefore such amount is not included here.

[4] 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.