Exit Costs - Transportation Commitments - Rollfoward of Accrued Transportation Commitment (Details) - Marcellus Shale Firm Transportation Agreement - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	$ 80	$ 90
Firm Transportation Exit Cost Accrual	92	0
Payments, Net of Accretion	(5)	(6)
Balance at End of Period	167	84
Other Current Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at End of Period	11	11
Accrued exit costs, current			$ 13	$ 13
Other Noncurrent Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at End of Period	$ 156	$ 73