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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, 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)

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