Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2019 USD ($)	Jul. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate			$ 33,034	$ 395,247
Related party transaction, rate			2.24%
Entity number of employees | employee			0
Expenses, related party			$ 395,300	370,400	$ 318,400
Severance Costs					7,400
Equity distributions			490,000	430,000	440,000
Cash contributions from parent			340,000	410,000	502,000
Duration Of Period For Deferred Revenue Recognition		25 years
Subsequent Event [Member]
Related Party Transaction [Line Items]
Equity distributions	$ 176,000
Hillabee Expansion Project [Member]
Related Party Transaction [Line Items]
Deferred Revenue, Additions		$ 240,000
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Operating revenues, related party			10,100	10,300	11,200
Cost of natural gas sales, related party			5,400	3,900	4,300
Expenses, related party			(4,700)	(3,700)	(4,300)
Williams Companies Inc [Member]
Related Party Transaction [Line Items]
Advances to affiliate			33,034	395,247
Duke Energy Corporation [Member]
Revenue, Major Customer [Line Items]
Operating revenues			194,500	198,400	178,900
National Grid [Member]
Revenue, Major Customer [Line Items]
Operating revenues			186,100	177,400	166,300
The Southern Company, Inc [Member]
Revenue, Major Customer [Line Items]
Operating revenues			$ 166,200	$ 160,000	$ 69,600