Derivative Instruments - Natural Gas Commodity Contracts Fair Value Hedges (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2016	Sep. 30, 2016
Effect of Fair Value Hedges on Results of Operations [Abstract]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	[1]	$ 0	$ (233)
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	[1]	0	681
Gain (Loss) on Fair Value Hedges Recognized in Earnings	[1]	0	448
Loss on Fair Value Hedge Ineffectiveness	[1]	0	(83)
Gain on Fair Value Hedge Ineffectiveness	[1]	0	531
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	[1]	$ 0	$ 448

[1] There were no natural gas futures commodity contracts designated as fair value hedges in 2015.