Debt Lines of Credit (Details) (USD $)	9 Months Ended		9 Months Ended
	Sep. 30, 2012 $1.25 billion LOC [Member]	Dec. 31, 2011 $1.25 billion LOC [Member]	Sep. 30, 2012 A 175 billion LOC [Member]	Dec. 31, 2010 $1.425 Billion LOC [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility		$ 1,250,000,000	$ 1,750,000,000	$ 1,425,000,000
Line of Credit Maturity			Jul. 13, 2014	Feb. 01, 2012
500.0 million ability to increase	500,000,000
Increase to Line of Credit	500,000,000
Advances under credit facility, rate plus spread			1.15%
Line of Credit Facility, Commitment Fee Percentage			0.20%
Remaining borrowing capacity	1,710,000,000
Amount restricted/dedicated to support letters of credit	30,200,000
Line of Credit Facility, Amount Outstanding	$ 7,000,000
Weighted Average Interest Rate Revolving Credit Facility			1.34%