Marcellus Shale Firm Transportation Commitments - Rollforward of Accrued Transportation Commitment (Details) - Marcellus Shale Firm Transportation and Other Obligations (1) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	$ 90	$ 0
Marcellus Exit Cost Accrual	0	93
Payments, Net of Accretion	(10)	(3)
Balance at End of Period	80	90
Other Current Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	14
Balance at End of Period	13	14
Other Noncurrent Liabilities
Rollforward Of Contractual Obligations [Roll Forward]
Balance at Beginning of Period	76
Balance at End of Period	$ 67	$ 76