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Employee Benefit Plans (Fair Value of Investments) (Details) - CAD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 100.00% | 100.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 2,408 | $ 2,208 |
Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 802 | 732 |
Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 132 | $ 114 |
Cash and cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 1.00% | 1.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 32 | $ 31 |
Cash and cash equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 32 | 31 |
Cash and cash equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Net In Transit [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | (1.00%) | (2.00%) |
Defined Benefit Plan, Fair Value of Plan Assets | $ (36) | $ (42) |
Net In Transit [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | (36) | (42) |
Net In Transit [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Net Asset Value Member [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1,474 | $ 1,362 |
Open Ended Invesments Measured at NAV [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 44.00% | 51.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1,065 | $ 1,132 |
Open Ended Invesments Measured at NAV [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Open Ended Invesments Measured at NAV [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Collective Trust Measured at NAV [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 18.00% | 11.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 409 | $ 230 |
Collective Trust Measured at NAV [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Collective Trust Measured at NAV [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Equity Securities, Canadian [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 9.00% | 9.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 214 | $ 192 |
Equity Securities, Canadian [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 214 | 192 |
Equity Securities, Canadian [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Equity Securities, United States [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 16.00% | 14.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 390 | $ 303 |
Equity Securities, United States [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 390 | 303 |
Equity Securities, United States [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Equity Securities, Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 8.00% | 11.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 197 | $ 243 |
Equity Securities, Other [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 197 | 243 |
Equity Securities, Other [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Government Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 3.00% | 2.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 72 | $ 47 |
Government Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Government Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 72 | $ 47 |
Corporate Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 2.00% | 2.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 56 | $ 53 |
Corporate Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Corporate Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 56 | $ 53 |
Other Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 0.00% | 1.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 5 | $ 19 |
Other Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 5 | 5 |
Other Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | 14 |
Other Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 0.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 4 |  |
Other Investments | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 |  |
Other Investments | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 4 |  |
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 2,408 | $ 2,208 | $ 1,300
Postretirement Benefit Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 100.00% | 100.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 45 | $ 39 | $ 6
Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1 | 1 |
Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 39 | $ 33 |
Postretirement Benefit Plan [Member] | Cash and cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 2.00% | 3.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1 | $ 1 |
Postretirement Benefit Plan [Member] | Cash and cash equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1 | 1 |
Postretirement Benefit Plan [Member] | Cash and cash equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 |
Postretirement Benefit Plan [Member] | Life insurance policies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 87.00% | 85.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 39 | $ 33 |
Postretirement Benefit Plan [Member] | Life insurance policies [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 |
Postretirement Benefit Plan [Member] | Life insurance policies [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 39 | 33 |
Postretirement Benefit Plan [Member] | Net Asset Value Member [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 5 | $ 5 |
Postretirement Benefit Plan [Member] | Other investments measured at Net Asset Value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 11.00% | 12.00% |
Defined Benefit Plan, Fair Value of Plan Assets | $ 5 | $ 5 |

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