Derivative Instruments - Schedule of Notional Amounts (Details) MWh in Thousands, MMBTU in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 MMBTU MWh	Dec. 31, 2019 MWh MMBTU
Non-trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	2,820	6,130
Non-trading | Natural Gas Basis | Buy
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	0	42
Non-trading | Electricity | Buy
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure) | MWh	3,479	6,015
Trading | Natural Gas | Buy
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	299	204
Trading | Natural Gas Basis | Sell
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	0	0