Risk Management Activities: Derivatives Not Designated as Hedge Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Regulatory assets	$ 286,707	$ 267,232	$ 284,235
Deferred Derivative Gain (Loss)
Derivative [Line Items]
Regulatory assets	3,900	11,000	$ 6,200
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Gain (Loss) on Derivative, Net	25	254
Not Designated as Hedging Instrument | Cost of Sales
Derivative [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 25	$ 254