Transactions with Affiliates (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 31, 2018 USD ($)	Jul. 31, 2017 USD ($)	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			$ 589,133		$ 589,133		$ 395,247
Related party transaction, rate					1.80%
Entity number of employees | employee			0		0
Expenses, related party			$ 99,300	$ 87,300	$ 190,800	$ 169,700
Cash distributions to parent					190,000	210,000
Cash contributions from parent					340,000	110,000
Noncash distribution to Parent		$ 240,000
Duration Of Period For Deferred Revenue Recognition		25 years
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Operating revenues, related party			1,500	3,100	3,300	6,400
Cost of natural gas sales, related party			1,800	800	3,700	1,900
Expenses, related party			(1,200)	$ (900)	(2,200)	$ (1,800)
Williams Partners L.P. [Member]
Related Party Transaction [Line Items]
Advances to affiliate			$ 589,100		$ 589,100		$ 395,200
Subsequent Event [Member]
Related Party Transaction [Line Items]
Cash distributions to parent	$ 100,000