Commitments and Contingencies - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended	24 Months Ended
	Mar. 31, 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	$ 84.0
Unutilized commitments expense	2.0
Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Commitment amount	1,400.0
Consent Decree
Other Commitments [Line Items]
Civil penalty		$ 5.0	$ 82.0
Mitigation projects	4.5
Supplemental environmental projects	$ 4.0
Minimum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Term	2 years
Maximum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Term	16 years