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Risk Management Activities: Cash Flow Hedges (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net $ (75) $ (953) $ 0
Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (2,271) 7,106 10,813
Designated as Hedging Instrument | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net 2,637 (38,901) 1,857
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net 0 (31,222) 2,888
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 2,941 3,899 3,647
Interest Rate Swap | Interest Expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net 0 (953) 0
Interest Rate Swap | Interest Expense | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (2,941) (3,899) (3,647)
Commodity Contract | Designated as Hedging Instrument | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net 366 (573) 9,782
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (670) (11,005) (14,460)
Commodity Contract | Discontinued Operations, Held-for-sale or Disposed of by Sale      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net 0 0 0
Commodity Contract | Discontinued Operations, Held-for-sale or Disposed of by Sale | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 913 11,019 $ 14,460
Commodity Contract | Fuel, purchased power and cost of natural gas sold      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net (75) 0  
Commodity Contract | Fuel, purchased power and cost of natural gas sold | Cash Flow Hedging      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ (243) $ (14)