Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	$ 34,257	$ 25,128
AOCI from business acquisition	0
Additions to AOCI	17,905	34,916
Amortization in current period	(3,530)	(1,074)
Loss on plan settlements		(2,547)
Reclassification to regulatory accounts (note 7(b))	(10,122)	(22,166)
Ending balance, December 31	38,510	34,257
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(6,221)	(4,995)
AOCI from business acquisition	(285)
Additions to AOCI	(7,705)	(1,875)
Amortization in current period	784	649
Gain (loss) on plan settlements		0
Reclassification to regulatory accounts (note 7(b))	7,247	0
Ending balance, December 31	(6,180)	(6,221)
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	(13,888)	(3,182)
AOCI from business acquisition	0
Additions to AOCI	14,871	3,254
Amortization in current period	409	272
Loss on plan settlements		0
Reclassification to regulatory accounts (note 7(b))	(10,538)	(14,232)
Ending balance, December 31	(9,146)	(13,888)
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(208)	(470)
AOCI from business acquisition	0
Additions to AOCI	0	0
Amortization in current period	208	262
Gain (loss) on plan settlements		0
Reclassification to regulatory accounts (note 7(b))	0	0
Ending balance, December 31	$ 0	$ (208)

Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period from Acquisition, before Tax

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Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) On Plan Settlements, Reclassification Adjustment from AOCI, before Tax

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Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment to Regulatory Accounts, before Tax

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Plan Settlements, Reclassification Adjustment from AOCI, before Tax

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Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit) from Acquisition, before Tax

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Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]

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Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for 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 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 reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of 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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