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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Defined benefit pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1,077 $ 1,262 $ 2,243 $ 2,684
Interest cost 3,571 6,159 8,891 12,300
Expected return on plan assets (5,752) (6,443) (11,485) (12,916)
Amortization of prior service cost 7 7 15 15
Recognized actuarial loss 1,821 1,626 3,641 3,462
Net periodic benefit cost 724 2,611 3,305 5,545
Postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0 13 1 26
Interest cost 151 286 406 572
Recognized actuarial loss 38 37 75 74
Net periodic benefit cost $ 189 $ 336 $ 482 $ 672