Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) employee	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate		$ 811,693	$ 64,608
Related party transaction, rate		0.39%
Entity number of employees | employee		0
Expenses, related party		$ 318,400	327,100	$ 310,100
Severance Costs		7,400
Equity distributions		440,000	536,000	411,000
Cash contributions from parent		502,000	652,000	267,000
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Operating revenues, related party		11,200	4,600	8,300
Purchases, related party		4,300	6,000	10,500
Expenses, related party		(4,300)	(5,700)	(6,600)
Williams Partners L.P. [Member]
Related Party Transaction [Line Items]
Advances to affiliate		811,700	64,600
Williams Field Services [Member]
Related Party Transaction [Line Items]
Reimbursements and pre-payments for capital projects				5,000
Acquired certain assets			1,900
Subsequent Event [Member]
Related Party Transaction [Line Items]
Equity distributions	$ 100,000
Cash contributions from parent	$ 110,000
National Grid [Member]
Revenue, Major Customer [Line Items]
Operating revenues		166,300	129,600	91,200
Public Service Enterprise Group [Member]
Revenue, Major Customer [Line Items]
Operating revenues		143,600	110,200	115,300
Duke Energy Corporation [Member]
Revenue, Major Customer [Line Items]
Operating revenues		$ 178,900	$ 94,600	$ 91,500