Derivative Instruments and Fair Value Measurements - Use of derivative instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative, Fair Value, Net [Abstract]
As reported fair value contracts	$ 162	$ 166
As reported fair value contracts	139	145
Commodity Contract \| Not Designated as Hedging Instrument \| Current Assets
Derivative, Fair Value, Net [Abstract]
Asset derivatives:	162	166
Liability derivatives:	0	0
Less counterparty offsets	0	0
As reported fair value contracts	162	166
Commodity Contract \| Not Designated as Hedging Instrument \| Other Assets
Derivative, Fair Value, Net [Abstract]
Asset derivatives:	0	0
Liability derivatives:	0	0
Less counterparty offsets	0	0
As reported fair value contracts	0	0
Commodity Contract \| Not Designated as Hedging Instrument \| Current Liabilities
Derivative, Fair Value, Net [Abstract]
Asset derivatives:	0	0
Liability derivatives:	139	145
Less counterparty offsets	0	0
As reported fair value contracts	139	145
Commodity Contract \| Not Designated as Hedging Instrument \| Other Liabilities
Derivative, Fair Value, Net [Abstract]
Asset derivatives:	0	0
Liability derivatives:	0	0
Less counterparty offsets	0	0
As reported fair value contracts	$ 0	$ 0

Fair value of liability associated with 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.

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Fair value, before 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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after effects of master netting arrangements, of 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 offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after effects of master netting arrangements, of financial liability or 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 offset against the right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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No definition available.

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- Definition

Current liabilities attributable to energy marketing activities (trading activities). Current energy marketing (trading activities) are when entities engage in both financial and physical trading to increase profits, manage their commodity price risk and enhance system reliability. They may trade electricity, coal, natural gas and oil. These entities typically use a variety of financial instruments, including forward contracts, options and swaps.

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Carrying amount as of the balance sheet date of the asset arising from energy trading activities that involves the purchase and sale of energy under forward contracts at fixed and variable prices and the buying and selling of financial energy contracts that include exchange futures and options and over the counter options and swaps, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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