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Employee Benefit Plans - Postretirement Healthcare Plan (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure        
Amortization of gain, weighted average remaining service period (in years)   4 years    
Stockholders’ equity $ 1,693,900 $ 1,427,600 $ 982,600 $ 950,500
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]        
Defined Benefit Plan, Amortization of Transition Asset (Obligation) $ 700      
Postretirement Healthcare Plan        
Defined Benefit Plan Disclosure        
Minimum age requirement 55 years      
Requisite service period 15 years      
Accumulated other comprehensive income (loss), before tax, related to the Plan $ 1,200      
Reduction in accumulated post-retirement benefit obligation due to curtailment     2,800  
Reduction in net periodic benefit cost due to curtailment (2,800) (300)    
Unfunded benefit obligation (500) (700)    
Net periodic benefit cost 800 $ 400 $ 200  
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]        
2019 120      
2020 90      
2021 60      
2022 50      
2023 through 2027 100      
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs $ 140      
Minimum        
Defined Benefit Plan Disclosure        
Assumptions used calculating net periodic benefit cost, discount rate 3.60% 2.40%    
Maximum        
Defined Benefit Plan Disclosure        
Assumptions used calculating net periodic benefit cost, discount rate 6.00% 6.00%