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Employee Future Benefits - Fair Value of Plan Assets by Major Category (Details) - CAD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | $ 12 | $ 13
Total | 381 | 428
Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 65 | 76
US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 26 | 31
International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 101 | 118
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 | 48 | 43
AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 64 | 71
A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 39 | 44
BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 22 | 26
Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 3 | 5
Level I |  |
Disclosure of fair value of plan assets [line items] |  |
Money market and cash and cash equivalents | (2) | (1)
Total | (1) | 0
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 | 14 | 14
Total | 381 | 427
Level II | Canadian |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 65 | 76
Level II | US |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 26 | 31
Level II | International |  |
Disclosure of fair value of plan assets [line items] |  |
Equity securities | 101 | 118
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 | 48 | 43
Level II | AA |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 64 | 71
Level II | A |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 39 | 44
Level II | BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 21 | 25
Level II | Below BBB |  |
Disclosure of fair value of plan assets [line items] |  |
Bonds | 3 | 5
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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