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Employee Future Benefits - Fair Value of Plan Assets by Major Category (Details) - CAD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | $ 19 | $ 12
Total | 386 | 381
Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 66 | 65
US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 28 | 26
International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 102 | 101
Private [Member] |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 1 | 1
AAA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 40 | 48
AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 68 | 64
A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 37 | 39
BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 22 | 22
Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 3 | 3
Level I |  |
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | 0 | (2)
Total | 1 | (1)
Level I | Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level I | US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level I | International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level I | Private [Member] |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level I | AAA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level I | AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level I | A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level I | BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 1 | 1
Level I | Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level II |  |
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | 19 | 14
Total | 384 | 381
Level II | Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 66 | 65
Level II | US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 28 | 26
Level II | International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 102 | 101
Level II | Private [Member] |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level II | AAA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 40 | 48
Level II | AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 68 | 64
Level II | A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 37 | 39
Level II | BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 21 | 21
Level II | Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 3 | 3
Level III |  |
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | 0 | 0
Total | 1 | 1
Level III | Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level III | US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level III | International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 0 | 0
Level III | Private [Member] |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 1 | 1
Level III | AAA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level III | AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level III | A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level III | BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 0 | 0
Level III | Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | $ 0 | $ 0

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