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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Employee Benefits Disclosure [Line Items]      
Defined contribution pension plan cost   $ 8,446 $ 7,232
Accumulated benefit obligation for pension plan   439,458 490,108
Actuarial (gain) loss $ (2,354)    
Gain on curtailment $ 853    
Defined benefit plan, amounts recognized in other comprehensive income (loss), net prior service cost (credit)   1,875  
Reclassification of non-service costs     9,035
Pension Plans      
Employee Benefits Disclosure [Line Items]      
Actuarial (gain) loss   (29,845) 35,696
Gain on curtailment   (1,875) (849)
Expected employer contributions for next year   20,137  
Other Postretirement Benefit Plans, Defined Benefit      
Employee Benefits Disclosure [Line Items]      
Actuarial (gain) loss   (14,800) 10,263
Gain on curtailment   0 $ (4)
Expected employer contributions for next year   $ 5,562