Marcellus Share Firm Transportation Contracts - Narrative (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MMBTU	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Purchased Oil and Gas		$ 76	$ 0	$ 204	$ 0
Committed transportation volume per day (in MMBtu) | MMBTU				450,000
Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Commitment amount				$ 1,500
Firm Transportation Accrual				$ 0		$ 93
Maximum | Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Term				15 years
Minimum | Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Term				4 years
Scenario, Forecast | Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Commitment amount	$ 925
Retained Transportation Contract | Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Firm Transportation Accrual						41
Future Commitments | Marcellus Shale Firm Transportation Agreement
Long-term Purchase Commitment [Line Items]
Firm Transportation Accrual						$ 52