Transactions with Major Customers and Affiliates (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) employee	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate		$ 64,608	$ 306,910
Related party transaction, rate		0.12%
Entity number of employees | employee		0
Expenses, related party		$ 327,100	310,100	$ 310,300
Equity distributions		536,000	411,000	250,000
Cash contributions from parent		652,000	267,000	264,000
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Operating revenues, related party		4,600	8,300	16,300
Purchases, related party		6,000	10,500	6,900
Expenses, related party		(5,700)	(6,600)	(7,100)
Williams Partners L.P. [Member]
Related Party Transaction [Line Items]
Advances to affiliate		64,600	306,900
Williams Field Services [Member]
Related Party Transaction [Line Items]
Reimbursements and pre-payments for capital projects			5,000	3,600
Acquired certain assets		1,900
Subsequent Event [Member]
Related Party Transaction [Line Items]
Equity distributions	$ 175,000
Cash contributions from parent	$ 112,000
National Grid [Member]
Revenue, Major Customer [Line Items]
Operating revenues		129,600	91,200	88,500
Public Service Enterprise Group [Member]
Revenue, Major Customer [Line Items]
Operating revenues		$ 110,200	$ 115,300	$ 130,700