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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	$ 25,128	$ 27,572
Additions to AOCI	34,916	(2,652)
Reclassification to regulatory accounts (note 7(b))	(22,166)	1,136
Amortization in current period	(1,074)	(928)
Gain (loss) on plan settlements	(2,547)
Ending balance, December 31	34,257	25,128
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(4,995)	(5,617)
Additions to AOCI	(1,875)	0
Reclassification to regulatory accounts (note 7(b))	0	0
Amortization in current period	649	622
Gain (loss) on plan settlements	0
Ending balance, December 31	(6,221)	(4,995)
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	(3,182)	(3,861)
Additions to AOCI	3,254	(3,066)
Reclassification to regulatory accounts (note 7(b))	(14,232)	3,515
Amortization in current period	272	230
Gain (loss) on plan settlements	0
Ending balance, December 31	(13,888)	(3,182)
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(470)	(732)
Additions to AOCI	0	0
Reclassification to regulatory accounts (note 7(b))	0	0
Amortization in current period	262	262
Gain (loss) on plan settlements	0
Ending balance, December 31	$ (208)	$ (470)

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

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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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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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).

+ References

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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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