Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details) MWh in Thousands, MMBTU in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 MWh MMBTU	Dec. 31, 2017 MWh MMBTU
Non-trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	5,644	9,191
Non-trading | Natural Gas Basis | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	143	0
Non-trading | Electricity | Buy
Derivatives, Fair Value [Line Items]
Net notional volume | MWh	7,008	8,091
Trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	117	26
Trading | Natural Gas Basis | Buy
Derivatives, Fair Value [Line Items]
Net notional volume	0
Trading | Natural Gas Basis | Sell
Derivatives, Fair Value [Line Items]
Net notional volume		225