Revolving Credit Facilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Revolving Credit Facilities [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 700,000,000
Credit Facilities - $370 million revolver	400,000,000
Credit Facilities - $300 million revolver	300,000,000
Line of Credit Facitlity, Covenant Terms, One Month Term	one
Line of Credit Facitlity, Covenant Terms, Two Month Term	two
Line of Credit Facitlity, Covenant Terms, Three Month Term	three
Line of Credit Facitlity, Covenant Terms, Six Month Term	six
Debt Instrument, Covenant Description	.65
Ratio of Indebtedness to Net Capital	0.489
Commercial Paper, Maximum Term	270
FERC Authorized Short-term Debt, effective through February 6, 2014	700,000,000
Short-term Debt	0
Commercial Paper	17,000,000	30,000,000
Line of Credit Facility, Amount Outstanding	67,000,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 616,000,000