Credit Facilities (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)	Apr. 09, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000,000
Potential maximum borrowing capacity	$ 600,000,000
Line of Credit Facitlity, Covenant Terms, Two Month Term	2 years
Debt Instrument, Covenant Description	65.00%
Ratio of Indebtedness to Net Capital	0.564
Commercial Paper, Maximum Term	270 days
Commercial Paper	$ 0
FERC Authorized Short-term Debt, effective through February 6, 2014	900,000,000
Line of Credit Facility, Amount Outstanding	0
Line of Credit Facility, Remaining Borrowing Capacity	500,000,000
letter of credit facility	220,000,000
Letters of Credit Outstanding, Amount	60,000,000
Short-term debt	$ 150,000,000		$ 0	$ 0
Line of Credit Facitlity, Covenant Terms, Six Month Term	6 months
Line of Credit Facitlity, Covenant Terms, One Month Term	1 year
Line of Credit Facitlity, Covenant Terms, Three Month Term	3 years
Term Loan [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 150,000,000