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Defined Benefit Plan (Details) - USD ($)
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Components of Net Periodic Benefit Cost [Abstract]      
Service cost $ 2,443,000 $ 1,479,000  
Interest cost 2,904,000 2,112,000  
Other cost 1,499,000 1,118,000  
Expected return on assets (4,120,000) (2,309,000)  
Net periodic benefit cost 2,726,000 $ 2,400,000  
Plan Assets [Abstract]      
Fair value of plan assets 220,659,000   $ 260,747,000
Employer Contributions [Abstract]      
Estimated employer contributions for the current fiscal year 9,238,000    
Defined Contribution Plan, Employer Discretionary Contribution Amount 0    
Quoted Prices in Active Markets for Identical Assets      
Plan Assets [Abstract]      
Fair value of plan assets 150,957,000   192,752,000
Significant Observable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 69,702,000   67,995,000
Significant Unobservable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 0   0
Cash and cash equivalents      
Plan Assets [Abstract]      
Fair value of plan assets 6,115,000   11,801,000
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets      
Plan Assets [Abstract]      
Fair value of plan assets 6,115,000   11,801,000
Cash and cash equivalents | Significant Observable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 0   0
Cash and cash equivalents | Significant Unobservable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 0   0
Actively Managed | Equity securities (a)      
Plan Assets [Abstract]      
Fair value of plan assets 122,326,000 [1]   157,050,000 [2]
Actively Managed | Equity securities (a) | Quoted Prices in Active Markets for Identical Assets      
Plan Assets [Abstract]      
Fair value of plan assets 116,326,000 [1]   149,265,000 [2]
Actively Managed | Equity securities (a) | Significant Observable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 6,000,000 [1]   7,785,000 [2]
Actively Managed | Equity securities (a) | Significant Unobservable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 0 [1]   0 [2]
Fixed Income | Fixed income (b)      
Plan Assets [Abstract]      
Fair value of plan assets 92,218,000 [3]   91,896,000 [4]
Fixed Income | Fixed income (b) | Quoted Prices in Active Markets for Identical Assets      
Plan Assets [Abstract]      
Fair value of plan assets 28,516,000 [3]   31,686,000 [4]
Fixed Income | Fixed income (b) | Significant Observable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets 63,702,000 [3]   60,210,000 [4]
Fixed Income | Fixed income (b) | Significant Unobservable Inputs      
Plan Assets [Abstract]      
Fair value of plan assets $ 0 [3]   $ 0 [4]
[1]
Actively managed portfolio of equity securities with the goal to exceed the benchmark performance
[2]
[3]
Actively managed portfolio of fixed income securities with the goal to exceed the benchmark performance
[4]