Derivatives and Risk Management - Schedule of Energy Commodity Derivative Volumes (Details) MWh in Thousands, MMBTU in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 MMBTU MWh	Dec. 31, 2020 MMBTU MWh
Purchase [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MWh	0	1
2022 | MWh	123	0
2023 | MWh	0	0
2024 | MWh	0	0
2025 | MWh	0	0
Purchase [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MMBTU	4,891	10,353
2022 | MMBTU	2,250	450
2023 | MMBTU	0	0
2024 | MMBTU	0	0
2025 | MMBTU	0	0
Purchase [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MWh	31	224
2022 | MWh	0	0
2023 | MWh	0	0
2024 | MWh	0	0
2025 | MWh	0	0
Purchase [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MMBTU	24,320	65,188
2022 | MMBTU	52,353	25,525
2023 | MMBTU	18,825	4,950
2024 | MMBTU	2,275	0
2025 | MMBTU	0	0
Sales [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MWh	18	17
2022 | MWh	216	0
2023 | MWh	0	0
2024 | MWh	0	0
2025 | MWh	0	0
Sales [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MMBTU	2,587	5,448
2022 | MMBTU	2,260	1,360
2023 | MMBTU	1,360	1,360
2024 | MMBTU	1,370	1,370
2025 | MMBTU	1,115	1,115
Sales [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MWh	103	451
2022 | MWh	370	0
2023 | MWh	0	0
2024 | MWh	0	0
2025 | MWh	0	0
Sales [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2021 | MMBTU	13,033	39,273
2022 | MMBTU	28,898	12,030
2023 | MMBTU	7,060	900
2024 | MMBTU	0	0
2025 | MMBTU	0	0