Fair Value Measurements: Balance Sheet Location (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Designated as Hedging Instrument
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Net	$ 129	$ 315	$ (423)
Not Designated as Hedging Instrument
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Net	745	197	(366)
Commodity Contract | Designated as Hedging Instrument | Derivative assets — current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	131	415	0
Commodity Contract | Designated as Hedging Instrument | Other assets, non-current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	9	18	0
Commodity Contract | Designated as Hedging Instrument | Derivative liabilities — current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(11)	(114)	(394)
Commodity Contract | Designated as Hedging Instrument | Other deferred credits and other liabilities
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0	(4)	(29)
Commodity Contract | Not Designated as Hedging Instrument | Derivative assets — current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	801	1,085	295
Commodity Contract | Not Designated as Hedging Instrument | Other assets, non-current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	46	1	0
Commodity Contract | Not Designated as Hedging Instrument | Derivative liabilities — current
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(84)	(833)	(497)
Commodity Contract | Not Designated as Hedging Instrument | Other deferred credits and other liabilities
Derivatives, Carrying Amount and Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (18)	$ (56)	$ (164)