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Defined Benefit Plan (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]          
Pension non-service cost $ 595 $ 1,032 $ 1,178 $ 1,907  
Components of Net Periodic Benefit Cost [Abstract]          
Service cost 1,607 1,288 3,203 2,614  
Interest cost 1,876 1,920 3,753 3,823  
Other cost 1,145 1,165 2,278 2,203  
Expected return on assets (2,426) (2,053) (4,853) (4,119)  
Net periodic benefit cost 2,202 2,320 4,381 4,521  
Plan Assets [Abstract]          
Fair value of plan assets 140,505   140,505   $ 140,164
Employer Contributions [Abstract]          
Capitalized pension non-service cost   265   265  
Estimated employer contributions for the current fiscal year 7,450   7,450    
Defined Contribution Plan, Employer Discretionary Contribution Amount     1,420    
Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 89,034   89,034   88,990
Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 51,471   51,471   51,174
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 $ 5,309   $ 5,309   8,207
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 5,309   5,309   8,207
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 6,450 [1]   6,450 [1]   6,413 [2]
Actively Managed | All Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 6,408 [1]   6,408 [1]   6,376 [2]
Actively Managed | All Cap Equity | Significant Observable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 42 [1]   42 [1]   37 [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 52,125 [1]   52,125 [1]   50,351 [2]
Actively Managed | U.S. Large Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 52,125 [1]   52,125 [1]   50,351 [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 9,628 [1]   9,628 [1]   9,358 [2]
Actively Managed | U.S. Mid Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 9,628 [1]   9,628 [1]   9,358 [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 9,805 [1]   9,805 [1]   8,725 [2]
Actively Managed | U.S. Small Cap Equity | Quoted Prices in Active Markets for Identical Assets          
Plan Assets [Abstract]          
Fair value of plan assets 9,805 [1]   9,805 [1]   8,725 [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 5,759 [1]   5,759 [1]   5,973 [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 5,759 [1]   5,759 [1]   5,973 [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 6,276 [1]   6,276 [1]   6,143 [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 6,276 [1]   6,276 [1]   6,143 [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 45,153 [3]   45,153 [3]   44,994 [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 45,153 [3]   45,153 [3]   44,994 [4]
Fixed Income | Significant Unobservable Inputs          
Plan Assets [Abstract]          
Fair value of plan assets 0 [3]   0 [3]   $ 0 [4]
ASU 2017-07          
Net Periodic Benefit Cost - Comprehensive Income Presentation [Abstract]          
Administrative and general 902 723 1,797 1,464  
Maintenance expense 279 229 557 465  
Pension non-service cost 595 1,032 1,178 1,907  
Components of Net Periodic Benefit Cost [Abstract]          
Other cost 426 336 849 685  
Net periodic benefit cost $ 2,202 $ 2,320 $ 4,381 $ 4,521  
[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.