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Pension and other post-retirement benefits - Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 CAD ($)
Change in plan assets |  |  |  |
Non-current assets (note 11) | $ 8,437 |  | $ 3,161 |
Pension Plans |  |  |  |
Change in projected benefit obligation |  |  |  |
Projected benefit obligation, beginning of year | 484,707 |  | 531,694 |
Projected benefit obligation assumed from business combination | 20,196 |  | 0 |
Modifications to plans |  | $ (7,705) |  | $ 0
Service cost | 12,351 |  | 15,481 |
Interest cost | 20,222 |  | 19,077 |
Actuarial (gain) loss | 65,443 |  | (29,986) |
Contributions from retirees | 0 |  | 0 |
Gain on curtailment | 0 |  | (1,875) |
Medicare Part D | 0 |  | 0 |
Benefits paid | (30,244) |  | (49,684) |
Projected benefit obligation, end of year | 564,970 |  | 484,707 |
Change in plan assets |  |  |  |
Fair value of plan assets, beginning of year | 339,099 |  | 403,945 |
Plan assets acquired in business combination | 8,004 |  | 0 |
Actual return on plan assets | 68,025 |  | (36,987) |
Employer contributions | 22,190 |  | 21,825 |
Medicare Part D subsidy receipts | 0 |  | 0 |
Benefits paid | (30,244) |  | (49,684) |
Fair value of plan assets, end of year | 407,074 |  | 339,099 |
Unfunded status | (157,896) |  | (145,608) |
Non-current assets (note 11) | 0 |  | 0 |
Current liabilities | (1,415) |  | (873) |
Non-current liabilities | (156,481) |  | (144,735) |
Net amount recognized | (157,896) |  | (145,608) |
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |
Change in projected benefit obligation |  |  |  |
Projected benefit obligation, beginning of year | 168,325 |  | 176,975 |
Projected benefit obligation assumed from business combination | 11,646 |  | 0 |
Modifications to plans |  | $ 0 |  | $ 0
Service cost | 4,587 |  | 5,791 |
Interest cost | 7,575 |  | 6,727 |
Actuarial (gain) loss | 33,605 |  | (14,800) |
Contributions from retirees | 1,913 |  | 1,878 |
Gain on curtailment | 0 |  | 0 |
Medicare Part D | 414 |  | 42 |
Benefits paid | (8,848) |  | (8,288) |
Projected benefit obligation, end of year | 219,217 |  | 168,325 |
Change in plan assets |  |  |  |
Fair value of plan assets, beginning of year | 115,542 |  | 130,487 |
Plan assets acquired in business combination | 15,688 |  | 0 |
Actual return on plan assets | 25,464 |  | (10,603) |
Employer contributions | 8,628 |  | 2,026 |
Medicare Part D subsidy receipts | 414 |  | 42 |
Benefits paid | (6,863) |  | (6,410) |
Fair value of plan assets, end of year | 158,873 |  | 115,542 |
Unfunded status | (60,344) |  | (52,783) |
Non-current assets (note 11) | 8,437 |  | 3,161 |
Current liabilities | (1,168) |  | (850) |
Non-current liabilities | (67,613) |  | (55,094) |
Net amount recognized | $ (60,344) |  | $ (52,783) |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 + Details
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