Fair Value of Derivative and Other Financial Instruments - Effect of Mark-to-Market Instruments on Earnings (Details) - Designated as hedging instrument [Member] - Commodity Contract [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	$ (1,261)	$ 6,863	$ 5,084
Electric operating revenues [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	(53)	7,156	4,491
Cost of energy [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	$ (1,208)	$ (293)	$ 593