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Employee Benefit Plans - Narrative (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
plan
SERPs  
Defined Benefit Plan Disclosure [Line Items]  
Number of non-qualified defined benefit plan | plan 2
Contributions to benefit plan $ 1.7
Estimated future contributions $ 4.8
Defined benefit plans  
Defined Benefit Plan Disclosure [Line Items]  
Number of non-qualified defined benefit plan | plan 2
Contributions to benefit plan $ 6.6
Estimated future contributions $ 10.8