Derivative Instruments - Gas hedges (Details) - Natural gas swaps $ in Thousands, MMBTU in Millions	12 Months Ended
Dec. 31, 2019 USD ($) MMBTU	Dec. 31, 2018 USD ($)
Derivative Instruments
Derivative liabilities \| $	$ 32,256	$ 13,154
Letters of credit \| $	$ 32,256
Notional volume of natural gas derivatives (in MMBTUs)	81.9
2020
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	25.1
2021
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	22.6
2022
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	15.5
2023
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	10.5
2024
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	8.2

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Nominal energy measure used to calculate payments on a derivative instrument.

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