Revolving Credit Facilities Revolving Credit Facilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Revolving Credit Facilities [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 670,000,000
Credit Facilities - $370 million revolver	370,000,000
Portion of $370 million credit facility scheduled to terminate in July 2012	10,000,000
Portion of $370 million credit facility scheduled to terminate in July 2013	360,000,000
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	0.515
Commercial Paper, Maximum Term	270
FERC Authorized Short-term Debt, effective through February 6, 2014	700,000,000
Short-term Debt	0
Commercial Paper	30,000,000	19,000,000
Line of Credit Facility, Amount Outstanding	124,000,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 516,000,000