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Pre-tax Components of Net Periodic Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Settlements, Benefit Obligation   $ 3,500  
Pension Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 2,069 1,562 $ 1,244
Interest cost 13,149 14,072 13,931
Expected return on plan assets (18,765) (19,389) (20,117)
Amortization of prior service cost (credit) 8 22 73
Amortization of actuarial loss 9,874 11,463 12,619
Defined Benefit Plan, Settlements, Benefit Obligation 0 3,482 0
Net periodic cost 6,335 11,212 7,750
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 163 249 363
Interest cost 2,745 3,075 3,881
Expected return on plan assets (1) (1) (1)
Amortization of prior service cost (credit) (1,535) (1,712) (2,178)
Amortization of actuarial loss (6,618) (7,566) 0
Defined Benefit Plan, Settlements, Benefit Obligation 0 0 0
Net periodic cost $ (5,246) $ (5,955) $ 2,065