Revolving Credit Facilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 700,000,000
Credit Facilities - $400 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	0.567
Commercial Paper, Maximum Term	270
FERC Authorized Short-term Debt, effective through February 6, 2014	900,000,000
Short-term Debt	0
Line of Credit Facility, Amount Outstanding	20,000,000
Line of Credit Facility, Remaining Borrowing Capacity	680,000,000
letter of credit facility	30,000,000
Letters of Credit Outstanding, Amount	$ 56,000,000