Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended		12 Months Ended			48 Months Ended
	Sep. 30, 2018 USD ($) well	Jul. 31, 2018 USD ($) well	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)
Other Commitments [Line Items]
Wells requiring corrective action | well		5
Wells alleged noncompliance | well	8
Settlement amount	$ 1,600	$ 135
Commitment amount			$ 1,500,000
Rental expense for office buildings and oil and gas operations equipment			90,000	$ 69,000	$ 76,000
Consent Decree
Other Commitments [Line Items]
Corrective actions			5,000			$ 84,000
Mitigation projects			4,000
Supplemental environmental projects			4,000
Consent Decree - Corrective Actions At Certain Tank Systems
Other Commitments [Line Items]
Corrective actions						$ 77,000
Other Transportation and Gathering Obligations
Other Commitments [Line Items]
Commitment amount			$ 612,000
Minimum
Other Commitments [Line Items]
Term			4 years
Minimum | Marcellus Shale Firm Transportation and Other Obligations (1)
Other Commitments [Line Items]
Term			2 years
Maximum
Other Commitments [Line Items]
Term			15 years
Maximum | Marcellus Shale Firm Transportation and Other Obligations (1)
Other Commitments [Line Items]
Term			10 years
Administrative Penalty Subject To Deferral Period
Other Commitments [Line Items]
Settlement amount		$ 41
Administrative Penalty Subject To Offset
Other Commitments [Line Items]
Settlement amount	$ 1,400