Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended			36 Months Ended
	May 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Other Commitments [Line Items]
Rental expense for office buildings and oil and gas operations equipment		$ 69,000,000	$ 76,000,000	$ 84,000,000
Consent Decree
Other Commitments [Line Items]
Civil penalty	$ 24,710			4,950,000	$ 72,000,000
Mitigation projects				4,500,000
Supplemental environmental projects	$ 98,840			$ 4,000,000
Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Long-term purchase commitment, amount		1,400,000,000
United States
Other Commitments [Line Items]
Long-term purchase commitment, amount		$ 781,000,000
Minimum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Commitment obligation, term		2 years
Minimum | United States
Other Commitments [Line Items]
Commitment obligation, term		1 year
Maximum | Marcellus Shale Firm Transportation Agreement
Other Commitments [Line Items]
Commitment obligation, term		16 years
Maximum | United States
Other Commitments [Line Items]
Commitment obligation, term		11 years