Derivative Instruments Gains (Loss) on Derivatives Recognized in Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financial Swaps | Off-system sales
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	[1]	$ (16)
Derivative, Gain on Derivative	[1]		$ 472	$ 1,379	$ 2,627
Financial Swaps | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	710	992	861	1,098
Financial Swaps | Fuel expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	[1]	(657)	(3,774)	(3,099)	(4,152)
Financial Swaps | Other operations and maintenance
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	[1]	(16)	(15)	(166)	(21)
Forward contracts | Purchased power
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	24	0	24	3
Forward contracts | Fuel expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	[1]	$ 49	$ 51	$ 139	$ 56

[1] Excludes unrealized gains or losses on derivatives, which are recorded on the balance sheet as regulatory assets or regulatory liabilities.