Transactions with Major Customers and Affiliates (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Feb. 28, 2021 USD ($)	Dec. 31, 2020 USD ($) employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Advances from affiliate	$ 252,549		$ 0	$ 252,549
Advances to affiliate	0		$ 642,734	0
Related party transaction, rate			0.01%
Entity number of employees | employee			0
Expenses, related party			$ 352,100	412,400	$ 395,300
Severance Costs				28,800
Cash distributions to parent			(1,020,000)	(824,000)	(490,000)
Cash contributions from parent			115,000	0	340,000
Non-cash distribution of investments in subsidiaries	36,600
Subsequent Event [Member]
Related Party Transaction [Line Items]
Cash contributions from parent		$ 60,000
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Operating revenues, related party			11,400	10,500	10,100
Cost of natural gas sales, related party			5,600	4,000	5,400
Expenses, related party			(5,100)	(4,400)	(4,700)
Williams Companies Inc [Member]
Related Party Transaction [Line Items]
Advances from affiliate			642,700
Advances to affiliate	$ 252,500			252,500
Cash distributions to parent			(1,020,000)	(824,000)	(490,000)
Atlantic Sunrise Project [Member]
Related Party Transaction [Line Items]
Additions to other financing obligations				25,000
Dominion Energy, Inc.
Revenue, Major Customer [Line Items]
Operating revenues			248,500	250,700	113,500
Duke Energy Corporation
Revenue, Major Customer [Line Items]
Operating revenues			232,800	222,500	194,500
The Southern Company, Inc.
Revenue, Major Customer [Line Items]
Operating revenues			$ 216,400	$ 216,100	$ 166,200