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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Derivative instruments: Volume activity of natural gas derivatives (Details) item in Millions	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Gas hedges
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 \| $	$ 24,995,000
Natural gas swaps
Gas hedges
Fair Value of liabilities \| $	$ 22,848,000	$ 18,914,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	54.7
Natural gas swaps \| 2016
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	23.1
Natural gas swaps \| 2017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	15.6
Natural gas swaps \| 2018
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	10.0
Natural gas swaps \| 2019
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	6.0