Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

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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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No definition available.

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Derivative Instruments - Notional volume of natural gas derivatives (Details) - Natural gas swaps item in Millions	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Gas hedges
Derivative liabilities \| $	$ 11,382,000	$ 6,328,000
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 \| $	$ 11,382,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	83.1
2018
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	15.4
2019
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	21.6
2020
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	18.4
2021
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	16.8
2022
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	10.7
2023
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	0.2