Acquisition (Details) - Gas Storage and Pipelines Acquisition - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 01, 2016	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Cash paid for gathering assets	$ 1,200
Assumed debt	204
Assumed debt, acquiree portion	$ 112
Noncontrolling interest percentage	45.00%
Assumed debt, noncontrolling interest portion	$ 92
Goodwill purchase accounting adjustments			$ 7
Useful life of acquired intangible assets	40 years
Pre-acquisition contingencies	$ 30
Direct transaction costs				$ 15
Operating revenues associated with acquired entities		$ 39
Net income (loss) associated with acquired entities		4
Transaction costs
Business Acquisition [Line Items]
Net income (loss) associated with acquired entities		$ (15)
Customer relationship intangibles
Business Acquisition [Line Items]
Intangible assets	$ 770
Useful life of acquired intangible assets	10 years
West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	55.00%		55.00%
M3 Midstream | Pennsylvania and West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	100.00%
M3 Midstream | West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	40.00%
Vega Energy Partners | West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	15.00%