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Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Regulatory Assets [Line Items]    
Regulatory Asset, Current $ 31,538 $ 38,721
Regulatory Asset, Noncurrent 362,943 346,683
Past expenditures not currently earning a return 64,000 65,000
Revaluation of Regulatory Assets for New Federal Tax Rate [Member]    
Regulatory Assets [Line Items]    
Regulatory Assets $ 23,000  
Pension and Retiree Medical Obligations    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Various  
Regulatory Asset, Current [1] $ 12,752 13,986
Regulatory Asset, Noncurrent [1] $ 223,038 234,171
Excess deferred taxes - TCJA    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Various  
Regulatory Asset, Current $ 0 0
Regulatory Asset, Noncurrent $ 44,685 0
Net AROs    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Plant lives  
Regulatory Asset, Current [2] $ 0 0
Regulatory Asset, Noncurrent [2] $ 24,201 24,352
Recoverable Deferred Taxes on AFUDC Recorded in Plant    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Plant lives  
Regulatory Asset, Current $ 0 [3] 0
Regulatory Asset, Noncurrent $ 23,888 [3] 44,258
Losses on Reacquired Debt    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Term of related debt  
Regulatory Asset, Current $ 807 127
Regulatory Asset, Noncurrent $ 22,664 1,617
Renewable Resources and Environmental Initiatives    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period One to three years  
Regulatory Asset, Current $ 1,600 3,580
Regulatory Asset, Noncurrent $ 1,301 2,900
Conservation Programs    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period One to two years  
Regulatory Asset, Current [4] $ 2,674 3,754
Regulatory Asset, Noncurrent [4] $ 733 2,431
Other Regulatory Assets    
Regulatory Assets [Line Items]    
Regulatory asset, remaining amortization period Various  
Regulatory Asset, Current $ 13,705 17,274
Regulatory Asset, Noncurrent $ 22,433 $ 36,954
[1] (a) Includes the non-qualified pension plan.
[2] (b) Includes amounts recorded for future recovery of AROs.
[3] (c) Includes a write-down of $23.2 million as a result of the revaluation of deferred tax gross up at the new federal tax rate at Dec. 31, 2017.
[4] (d) Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.