Derivative Instruments - Volumetric Underlying Derivative Transactions (Details) - Buy MWh in Thousands, MMBTU in Thousands	12 Months Ended
	Dec. 31, 2020 MMBTU MWh	Dec. 31, 2019 MMBTU MWh
Non-trading | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume	2,880	6,130
Non-trading | Natural Gas Basis
Derivatives, Fair Value [Line Items]
Net notional volume	0	42
Non-trading | Electricity
Derivatives, Fair Value [Line Items]
Net notional volume | MWh	1,845	6,015
Trading | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume	102	204