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Pension Plan and Other Postretirement Benefits - Funded Status (Details) - Pension Plan - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Accumulated Benefit Obligation $ (385,302) $ (346,723)  
Projected Benefit Obligation (428,396) (384,785) $ (328,442)
Fair Value of Plan Assets 360,678 329,781 $ 269,783
Funded Status $ (67,718) $ (55,004)