Marcellus Shale Firm Transportation Commitments - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Commitment amount		$ 1,500
Purchased Oil and Gas		296	$ 0	$ 0
Marcellus Shale Firm Transportation and Other Obligations (1)
Long-term Purchase Commitment [Line Items]
Marcellus Exit Cost Accrual		$ 0	93
Appalachian Gateway Project
Long-term Purchase Commitment [Line Items]
Exit costs			41
Leach/Rayne Xpress
Long-term Purchase Commitment [Line Items]
Exit costs			$ 52
Minimum
Long-term Purchase Commitment [Line Items]
Term		4 years
Minimum | Marcellus Shale Firm Transportation and Other Obligations (1)
Long-term Purchase Commitment [Line Items]
Term		2 years
Maximum
Long-term Purchase Commitment [Line Items]
Term		15 years
Maximum | Marcellus Shale Firm Transportation and Other Obligations (1)
Long-term Purchase Commitment [Line Items]
Term		10 years
Subsequent Event | Leach/Rayne Xpress
Long-term Purchase Commitment [Line Items]
Exit costs	$ 92
Commitment reduction	$ 350