Commitments and Contingencies Commitments and Contingencies - Additional Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Long-term Purchase Commitment [Line Items]
Purchase Obligation Agreement Expiration Date	2028
Future Drilling Commitments At Balance Sheet Date	$ 113
Drilling Commitments Year One	35
Drilling Commitments Year Two	77
Drilling Commitments Year Three	1
Purchase Obligation	1,288
Purchase Obligation, Future Minimum Payments, Remainder of Fiscal Year	59
Purchase Obligation, Due in Second Year	223
Purchase Obligation, Due in Third Year	191
Purchase Obligation, Due in Fourth Year	173
Purchase Obligation, Due in Fifth Year	165
Purchase Obligation, Due after Fifth Year	$ 477
Future Drilling Commitments End Date	2020-02