Pensions and Other Benefits Pensions and Other Benefits - Company Contributions to Pension Plans (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contributions to Pension Plans [Line Items]
Total Company pension contribution (DB and DC)	CAD 55	CAD 57	CAD 90
Pension Plans, Defined Benefit and Defined Contribution
Contributions to Pension Plans [Line Items]
Contribution by the Company (DB plans)	46	48
Contribution by the Company (DC Plans)	9	9	9
Other Pension Plans, Postretirement or Supplemental Plans, Defined Benefit [Member]
Contributions to Pension Plans [Line Items]
Contribution by the Company (DB plans)	13	12	12
Canadian registered and U.S. qualified DB pension plans | Pension Plans, Defined Benefit and Defined Contribution
Contributions to Pension Plans [Line Items]
Contribution by the Company (DB plans)	CAD 33	CAD 36	CAD 69