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Defined Benefit Plan (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]          
Other production expenses $ 370 $ 426 $ 740 $ 849  
Administrative and general expense 851 902 1,689 1,797  
Maintenance expense 271 279 542 557  
Pension non-service costs 907 595 1,828 1,178  
Components of Net Periodic Benefit Cost [Abstract]          
Service cost 1,479 1,607 2,958 3,203  
Interest cost 2,113 1,876 4,225 3,753  
Other cost 1,117 1,145 2,235 2,278  
Expected return on assets (2,310) (2,426) (4,619) (4,853)  
Net periodic benefit cost 2,399 $ 2,202 4,799 $ 4,381  
Plan Assets [Abstract]          
Fair value of plan assets 152,539   152,539   $ 133,459
Employer Contributions [Abstract]          
Estimated employer contributions for the current fiscal year 8,337   8,337    
Defined Contribution Plan, Employer Discretionary Contribution Amount 2,294        
Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 97,878   97,878   82,677
Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 54,661   54,661   50,782
Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets $ 0   $ 0   0
Equity Securities [Member]          
Plan Assets [Abstract]          
Target plan asset allocations 55.00%   55.00%    
Fixed Income Securities          
Plan Assets [Abstract]          
Target plan asset allocations 45.00%   45.00%    
Cash and cash equivalents          
Plan Assets [Abstract]          
Fair value of plan assets $ 7,723   $ 7,723   8,136
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 7,723   7,723   8,136
Cash and cash equivalents | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0   0   0
Cash and cash equivalents | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0   0   0
Actively Managed | All Cap Equity          
Plan Assets [Abstract]          
Fair value of plan assets 6,461 [1]   6,461 [1]   5,670 [2]
Actively Managed | All Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 6,426 [1]   6,426 [1]   5,632 [2]
Actively Managed | All Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 35 [1]   35 [1]   38 [2]
Actively Managed | All Cap Equity | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Large Cap Equity          
Plan Assets [Abstract]          
Fair value of plan assets 57,375 [1]   57,375 [1]   47,040 [2]
Actively Managed | U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 57,375 [1]   57,375 [1]   47,040 [2]
Actively Managed | U.S. Large Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Large Cap Equity | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Mid Cap Equity          
Plan Assets [Abstract]          
Fair value of plan assets 10,202 [1]   10,202 [1]   8,372 [2]
Actively Managed | U.S. Mid Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 10,202 [1]   10,202 [1]   8,372 [2]
Actively Managed | U.S. Mid Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Mid Cap Equity | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Small Cap Equity          
Plan Assets [Abstract]          
Fair value of plan assets 10,385 [1]   10,385 [1]   8,528 [2]
Actively Managed | U.S. Small Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 10,385 [1]   10,385 [1]   8,528 [2]
Actively Managed | U.S. Small Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | U.S. Small Cap Equity | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | Non-U.S. Large Cap Equity          
Plan Assets [Abstract]          
Fair value of plan assets 5,767 [1]   5,767 [1]   4,969 [2]
Actively Managed | Non-U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 5,767 [1]   5,767 [1]   4,969 [2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | Non-U.S. Large Cap Equity | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | REIT          
Plan Assets [Abstract]          
Fair value of plan assets 7,137 [1]   7,137 [1]   5,889 [2]
Actively Managed | REIT | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Actively Managed | REIT | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 7,137 [1]   7,137 [1]   5,889 [2]
Actively Managed | REIT | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [1]   0 [1]   0 [2]
Fixed Income          
Plan Assets [Abstract]          
Fair value of plan assets 47,489 [3]   47,489 [3]   44,855 [4]
Fixed Income | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 0 [3]   0 [3]   0 [4]
Fixed Income | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 47,489 [3]   47,489 [3]   44,855 [4]
Fixed Income | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets $ 0 [3]   $ 0 [3]   $ 0 [4]
[1]
Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.
[2]
Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.
[3]
Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.
[4]
Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.