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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Derivative instruments: - Volume activity of natural gas derivatives (Details) item in Millions	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Gas hedges
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 \| $	$ 62,000
Natural gas swaps
Gas hedges
Derivative assets \| $	15,090,000
Derivative liabilities \| $	$ 22,848,000	$ 22,848,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	68.6
Natural gas swaps \| 2017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	23.2
Natural gas swaps \| 2018
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20.4
Natural gas swaps \| 2019
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	13.5
Natural gas swaps \| 2020
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	9.0
Natural gas swaps \| 2021
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	2.5