Debt Lines of Credit (Details) (USD $)	3 Months Ended
	Mar. 31, 2013
Line of Credit Facility [Line Items]
Debt Instrument, Maturity Date	Sep. 01, 2014
500.0 million ability to increase	$ 500,000,000
A 175 billion LOC [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	1,750,000,000
Debt Instrument, Maturity Date	Jul. 01, 2014
A 250 billion LOC [Member] [Domain]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	2,500,000,000
Debt Instrument, Maturity Date	Apr. 01, 2018
Remaining borrowing capacity	2,070,000,000
Amount restricted/dedicated to support letters of credit	36,700,000
Line of Credit Facility, Amount Outstanding	$ 395,000,000
Weighted Average Interest Rate Revolving Credit Facility	1.24%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line Of Credit Spread	1.05%