Basis of Presentation - Performance Obligation, Expected Timing (Details) $ in Millions	Mar. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Natural Gas Revenues	$ 224
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Expected timing	9 months
Natural Gas Revenues	$ 108
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Expected timing	1 year
Natural Gas Revenues	$ 116