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Employee Benefit Plans - Accumulated and Projected Benefit Obligations (Details) - Defined benefit pension plans - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Projected benefit obligation $ 577,418 $ 545,367
Accumulated benefit obligation 559,039 525,661
Fair value of plan assets $ 351,650 $ 334,604