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Employee Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 0 $ 1,307 $ 1,484
Interest cost 3,259 3,474 3,319
Expected return on plan assets (3,781) (4,030) (4,027)
Amortization of prior service cost 0 0 13
Amortization of losses (gains) 466 2,526 3,733
Net periodic benefit (credit) cost (56) 3,277 4,522
Postretirement      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 6 4 11
Interest cost 53 42 52
Expected return on plan assets 0 0 0
Amortization of prior service cost 0 0 0
Amortization of losses (gains) (1) (13) 12
Net periodic benefit (credit) cost $ 58 $ 33 $ 75