Derivative Instruments and Risk Management Activities - Assets and Liabilities Associated with Interest Rate Derivatives (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets from interest rate risk management activities:
Other long-term assets	$ 18,548	$ 9,501
Total	18,548	9,501
Liabilities from interest rate risk management activities:
Current	682	1,996
Long-term	1,597	8,583
Total	$ 2,279	$ 10,579

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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