Credit Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500
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	0.65
Ratio of Indebtedness to Net Capital	0.515
Commercial Paper, Maximum Term	270
Commercial Paper	$ 0
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	220
Letters of Credit Outstanding, Amount	$ 84