Derivative Instruments - Gas hedges (Details) - Natural gas swaps MMBTU in Millions	9 Months Ended
Sep. 30, 2019 USD ($) MMBTU	Dec. 31, 2018 USD ($)
Derivative Instruments
Derivative liabilities \| $	$ 26,379,000	$ 13,154,000
Collateral or letters of credit required to be posted with counterparties, if credit-risk-related contingent features were triggered due to credit rating being downgraded below investment grade \| $	$ 26,379,000
Notional volume of natural gas derivatives (in MMBTUs)	83.1
2019
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	3.5
2020
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	24.9
2021
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	22.0
2022
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	15.3
2023
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	10.2
2024
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	7.2

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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