Commitments and Contingencies - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended	24 Months Ended
	Jun. 30, 2018 USD ($) MMBTU	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Committed transportation volume per day (in MMBtu) | MMBTU	450,000
Firm transportation liability	$ 83
Unutilized commitments expense	3
Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Commitment amount	1,400
Consent Decree
Other Commitments [Line Items]
Civil penalty		$ 5	$ 83
Mitigation projects	5
Supplemental environmental projects	$ 4
Minimum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Term	4 years
Maximum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Term	15 years