Marcellus Share Firm Transportation Contracts - Rollfoward of Accrued Transportation Commitment (Details) - Marcellus Shale Firm Transportation Agreement - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	$ 90	$ 0
Firm Transportation Accrual	0	93
Payments	(9)	(3)
Balance at End of Period	81	90
Other Current Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	14
Balance at End of Period	12	14
Other Noncurrent Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	76
Balance at End of Period	$ 69	$ 76