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Employee Benefits - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]        
Service costs included in net periodic pension expense     $ 0  
Pension contribution   $ 20,400,000    
Expected long-term rate of return on plan assets     4.80% 7.50%
Return to provision discrete tax benefit due to decrease in federal statutory tax rate $ 2,900,000      
Fixed Income Funds | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation (as a percentage)     85.00% 30.00%
Fixed Income Funds | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation (as a percentage)     95.00% 50.00%
Equity Securities | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation (as a percentage)     5.00% 50.00%
Equity Securities | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation (as a percentage)     15.00% 70.00%