Fair Value of Derivative and Other Financial Instruments, Statement of Earnings Information (Details) - Designated as hedging instrument [Member] - Commodity Contract [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	$ 6,863	$ 5,084	$ 2,836
Electric operating revenues [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	7,156	4,491	1,727
Cost of energy [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss)	$ (293)	$ 593	$ 1,109