Credit Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500
Line of Credit Facitlity, Covenant Terms, One Month Term	1
Line of Credit Facitlity, Covenant Terms, Two Month Term	2
Line of Credit Facitlity, Covenant Terms, Three Month Term	3
Line of Credit Facitlity, Covenant Terms, Six Month Term	6
Debt Instrument, Covenant Description	0.65
Ratio of Indebtedness to Net Capital	0.510
Commercial Paper, Maximum Term	270
Commercial Paper	$ 0	$ 6
FERC Authorized Short-term Debt, effective through February 6, 2014	900
Line of Credit Facility, Amount Outstanding	0
Line of Credit Facility, Remaining Borrowing Capacity	500
letter of credit facility	160
Letters of Credit Outstanding, Amount	$ 56