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Employee Benefit Plans (Fair Value of Investments) (Details) - CAD CAD in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
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 | CAD 2,208 | CAD 1,301 |
Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 732 | CAD 682 |
Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 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 | CAD 31 | CAD 12 |
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 | 31 | CAD 12 |
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 | CAD 0 |  |
Net In Transit [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | (2.00%) |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD (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 | (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 |  |
Net Asset Value Member [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 1,362 |  |
Open Ended Invesments Measured at NAV [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 51.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 1,132 |  |
Collective Trust Measured at NAV [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 11.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 230 |  |
Other investments measured at Net Asset Value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 48.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | CAD 619 |
Other investments measured at Net Asset Value [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | CAD 0 |
Equity Securities, Canadian [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 9.00% | 0.00% |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 192 | CAD 190 |
Equity Securities, Canadian [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 192 | CAD 190 |
Equity Securities, United States [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 14.00% | 18.00% |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 303 | CAD 240 |
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 | CAD 303 | CAD 240 |
Equity Securities, Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 11.00% | 18.00% |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 243 | CAD 240 |
Equity Securities, Other [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 243 | 240 |
Government Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 2.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 47 |  |
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 | CAD 47 |  |
Corporate Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 2.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 53 |  |
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 | CAD 53 |  |
Other Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 9.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 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 |  |
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 | 14 |  |
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 2,208 | 1,300 | CAD 1,205
Postretirement Benefit Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 100.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 39 | CAD 6 | CAD 5
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 |  |
Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 33 |  |
Postretirement Benefit Plan [Member] | Cash and cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 3.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 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 |  |
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 | CAD 0 |  |
Postretirement Benefit Plan [Member] | Life insurance policies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 85.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 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 |  |
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 | 33 |  |
Postretirement Benefit Plan [Member] | Net Asset Value Member [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 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 | 12.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 5 |  |
Post Retirement Benefit Plans |  |  |
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 | CAD 39 | CAD 5 |
Post Retirement Benefit Plans | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1 | 1 |
Post Retirement Benefit Plans | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 33 | CAD 0 |
Post Retirement Benefit Plans | Cash and cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 3.00% | 20.00% |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 1 | CAD 1 |
Post Retirement Benefit Plans | 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 |
Post Retirement Benefit Plans | 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 | CAD 0 | 0 |
Post Retirement Benefit Plans | Life insurance policies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 85.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 33 |  |
Post Retirement Benefit Plans | 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 |  |
Post Retirement Benefit Plans | Life insurance policies [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 33 |  |
Post Retirement Benefit Plans | Net Asset Value Member [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 5 | CAD 4 |
Post Retirement Benefit Plans | Other investments measured at Net Asset Value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations | 12.00% | 80.00% |
Defined Benefit Plan, Fair Value of Plan Assets | CAD 5 | CAD 4 |

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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