Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate		$ 395,247	$ 811,693
Related party transaction, rate		1.16%
Entity number of employees | employee		0
Expenses, related party		$ 370,400	318,400	$ 327,100
Severance Costs			7,400
Equity distributions		430,000	440,000	536,000
Cash contributions from parent		410,000	502,000	652,000
Duration Of Period For Deferred Revenue Recognition	25 years
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		10,300	11,200	4,600
Purchases, related party		3,900	4,300	6,000
Expenses, related party		3,700	(4,300)	(5,700)
Williams Partners L.P. [Member]
Related Party Transaction [Line Items]
Advances to affiliate		395,200	811,700
Duke Energy Corporation [Member]
Revenue, Major Customer [Line Items]
Operating revenues		198,400	178,900	94,600
National Grid [Member]
Revenue, Major Customer [Line Items]
Operating revenues		177,400	166,300	129,600
The Southern Company, Inc [Member]
Revenue, Major Customer [Line Items]
Operating revenues		$ 160,000	$ 69,600	$ 61,600