Basis of Presentation - Performance Obligation, Expected Timing (Details) $ in Millions	Sep. 30, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-12-31
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing	3 months
Natural Gas Revenues	$ 54
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-12-31
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing	2 years
Natural Gas Revenues	$ 137
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-12-31
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing	3 years
Natural Gas Revenues	$ 360