Acquisition (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 01, 2016	Dec. 31, 2016	Jun. 30, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 2,286	$ 2,291	$ 2,286
Gas Storage and Pipelines Acquisition
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	273
Escrow deposit	$ 130		28
Goodwill purchase accounting adjustments			$ 5
Useful life of acquired intangible assets	40 years
Pre-acquisition contingencies	$ 39
Direct transaction costs				$ 15
Operating revenues associated with acquired entities		39
Net income (loss) associated with acquired entities		4
Gas Storage and Pipelines Acquisition | Transaction costs
Business Acquisition [Line Items]
Net income (loss) associated with acquired entities		$ (15)
Gas Storage and Pipelines Acquisition | Customer relationship intangibles
Business Acquisition [Line Items]
Intangible assets	$ 770
Useful life of acquired intangible assets	10 years
Gas Storage and Pipelines Acquisition | West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	55.00%		55.00%
Gas Storage and Pipelines Acquisition | M3 Midstream | Pennsylvania and West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	100.00%
Gas Storage and Pipelines Acquisition | M3 Midstream | West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	40.00%
Gas Storage and Pipelines Acquisition | Vega Energy Partners | West Virginia
Business Acquisition [Line Items]
Percent of assets acquired	15.00%