Derivative Instruments - Schedule of Notional Amounts (Details) - Buy MWh in Thousands, MMBTU in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 MMBTU MWh	Dec. 31, 2019 MMBTU MWh
Non-trading | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	2,484	6,130
Non-trading | Natural Gas Basis
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	0	42
Non-trading | Electricity
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure) | MWh	2,391	6,015
Trading | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume (energy measure)	134	204