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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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.

+ References

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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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No definition available.

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Derivative Instruments - Volume activity of natural gas derivatives (Details) - Natural gas swaps item in Millions	Mar. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Gas hedges
Derivative liabilities \| $	$ 25,919,000	$ 22,848,000
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 \| $	$ 25,919,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	67.6
2016
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	21.6
2017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	18.1
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)	10.0
2020
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	2.5