Derivative Instruments Fair Value Hedges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	[1]
Fair Value Hedges [Abstract]
Commodity contracts	$ (233)
Fair value adjustment for natural gas inventory designated as the hedged item	681
Total increase in purchased gas cost	448
Basis ineffectiveness	(83)
Timing ineffectiveness	531
Total ineffectiveness	$ 448

[1] (1) There were no natural gas futures commodity contracts designated as fair value hedges in 2017.