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Pension and Other Postretirement Employee Benefit Plans - Components of Net Periodic Cost of Pension and Other Postretirement Employee Benefit Plans (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]        
Non-operating pension and other postretirment benefit (costs) income $ 1,531 $ 1,187 $ 2,845 $ 2,466
Pension Benefit Plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Service Cost 669 461 1,219 895
Interest cost 3,102 3,010 6,223 6,010
Expected return on plan assets (4,132) (4,247) (8,267) (8,501)
Amortization of prior service cost (credit) 0 0 0 0
Amortization of actuarial loss 1,776 2,458 3,685 5,028
Net periodic cost 1,415 1,682 2,860 3,432
Other Postretirement Employee Benefit Plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Service Cost 20 20 45 68
Interest cost 752 611 1,399 1,218
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (credit) 0 (419) 0 (838)
Amortization of actuarial loss 33 (226) (195) (451)
Net periodic cost $ 805 $ (14) $ 1,249 $ (3)