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Employee Benefit Plans Employee Benefit Plans 3 (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Amounts recognized on the balance sheets:          
Non-current liabilities       $ 49,856 $ 42,666
Pension Benefits          
Amounts recognized on the balance sheets:          
Non-current assets       0 0
Current liabilities       0 0
Non-current liabilities       29,492 26,760
Net amount recognized       (29,492) (26,760)
Amounts recognized in regulatory assets consist of:          
Initial net obligation       0 0
Prior service cost (credit)       0 49
Net (gain) loss       (25,828) (21,921)
Regulatory assets (liabilities) $ 21,970 $ 39,170 $ 39,170 25,828 21,970
Unfunded accrued pension cost       (3,664) (4,790)
Net liability (asset) recognized       29,492 26,760
Changes in plan assets and benefit obligations recognized in regulatory assets:          
Regulatory asset at beginning of year 21,970 39,170      
Net loss (gain) (4,818) 15,292      
Amortization of initial net obligation 0 0      
Amortization of prior service (cost) credit (49) (118)      
Amortization of net gain (loss) (911) (1,790)      
Total change in regulatory asset 3,858 (17,200)      
Regulatory asset (liability) at end of year 25,828 21,970 39,170    
Net periodic pension costs 4,126 6,075 4,383    
Change in regulatory asset 3,858 (17,200)      
Total recognized in net periodic pension cost and regulatory asset (liability) 7,984 (11,125)      
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:          
Prior service (cost) credit 0 (49)      
Net gain (loss) (835) (510)      
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:          
Projected benefit obligation       180,364 168,934
Accumulated benefit obligation       165,998 155,469
Fair value of plan assets       $ 150,872 $ 142,174
Weighted-average assumptions used to determine benefit obligations at December 31:          
Discount rate       4.44% 4.65%
Post-Retirement Medical Benefits          
Amounts recognized on the balance sheets:          
Non-current assets       $ 1,736 $ 1,221
Current liabilities       0 0
Non-current liabilities       0 0
Net amount recognized       1,736 1,221
Amounts recognized in regulatory assets consist of:          
Initial net obligation       0 0
Prior service cost (credit)       0 (34)
Net (gain) loss       5,515 5,572
Regulatory assets (liabilities) (5,606) (3,125) (3,125) (5,515) (5,606)
Unfunded accrued pension cost       (3,779) (4,385)
Net liability (asset) recognized       (1,736) (1,221)
Changes in plan assets and benefit obligations recognized in regulatory assets:          
Regulatory asset at beginning of year (5,606) (3,125)      
Net loss (gain) 644 2,997      
Amortization of initial net obligation 0 0      
Amortization of prior service (cost) credit 34 200      
Amortization of net gain (loss) 701 316      
Total change in regulatory asset 91 (2,481)      
Regulatory asset (liability) at end of year (5,515) (5,606) (3,125)    
Net periodic pension costs (606) (234) $ 278    
Change in regulatory asset 91 (2,481)      
Total recognized in net periodic pension cost and regulatory asset (liability) (515) (2,715)      
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:          
Prior service (cost) credit 0 34      
Net gain (loss) $ 679 $ 599      
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:          
Projected benefit obligation       8,802 9,393
Fair value of plan assets       $ 10,538 $ 10,614
Weighted-average assumptions used to determine benefit obligations at December 31:          
Discount rate       3.97% 4.25%
Minimum | Pension Benefits          
Weighted-average assumptions used to determine benefit obligations at December 31:          
Rate of compensation increase       3.00%  
Maximum | Pension Benefits          
Weighted-average assumptions used to determine benefit obligations at December 31:          
Rate of compensation increase       8.00%