Transactions with Affiliates (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Advances to affiliate	$ 0		$ 0		$ 137,666
Advances from affiliate	$ 90,069		$ 90,069		0
Related party transaction, rate			1.80%
Entity number of employees | employee	0		0
Related party transaction, expenses from transactions with related party	$ 23,600	$ 23,200	$ 45,700	$ 46,200
Cash distributions to parent			87,000	$ 107,000
Williams Partners L.P. [Member]
Related Party Transaction [Line Items]
Advances to affiliate	0		0		$ 137,666
Advances from affiliate	$ 90,069		$ 90,069