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Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]        
Service cost, net $ 4,025 $ 4,149 $ 12,233 $ 12,509
Interest cost on projected benefit obligations 3,256 3,381 9,764 9,630
Expected return on plan assets (9,466) (9,594) (28,605) (27,847)
Recognition of prior service cost (1,820) (1,806) (5,527) (5,024)
Recognition of actuarial losses/(gains) 6,471 6,841 19,677 20,152
Net periodic pension cost/(credit) 2,466 2,971 7,542 9,420
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost, net 273 141 817 423
Interest cost on projected benefit obligations 1,060 1,094 3,182 3,282
Expected return on plan assets (1,733) (1,684) (5,197) (5,052)
Recognition of prior service cost 0 0 0 0
Recognition of actuarial losses/(gains) 1,451 1,639 4,353 4,917
Net periodic pension cost/(credit) 1,051 1,190 3,155 3,570
Other Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost, net 3,752 4,008 11,416 12,086
Interest cost on projected benefit obligations 2,180 2,269 6,534 6,294
Expected return on plan assets (7,733) (7,910) (23,408) (22,795)
Recognition of prior service cost (1,727) (1,611) (5,248) (4,439)
Recognition of actuarial losses/(gains) 5,331 5,676 16,261 16,657
Net periodic pension cost/(credit) 1,803 2,432 $ 5,555 7,803
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year, Description     25900  
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost, net 0 0 $ 0 0
Interest cost on projected benefit obligations 16 18 48 54
Expected return on plan assets 0 0 0 0
Recognition of prior service cost (93) (195) (279) (585)
Recognition of actuarial losses/(gains) (311) (474) (937) (1,422)
Net periodic pension cost/(credit) $ (388) $ (651) $ (1,168) $ (1,953)