Retirement plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive Income (Loss) [Line Items]
Current year net actuarial (loss) gain	$ (19,279)	$ 20,373	$ (21,337)
Pension and other postretirement benefit items	(6,640)	29,210	$ (13,269)
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial (loss) gain	(19,817)	16,272
Amortization of previously deferred actuarial loss	5,124	8,357
Amortization of previously deferred prior service costs	168	635
Pension payment timing related charges	7,498	0
Curtailment gain	0	4,716
Prior service cost recognized in curtailment	0	26
Pension and other postretirement benefit items	$ (7,027)	$ 30,006

Comprehensive Income (Loss) [Line Items]

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Pension Payment Timing Related Charges, Before Tax

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost, Curtailment Settlement Adjustment, before Tax

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of increase (decrease) to other comprehensive income from settlement and curtailment gain (loss) of defined benefit plan.

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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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