Post-employee benefit plans - Components of Post-Employment Benefit (Obligations) Assets (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2017
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	$ (1,846)
Interest on obligations	(69)	$ (72)
Actuarial gains (losses)	79	(313)
Post-employment benefit obligations/Fair value of plan assets, December 31	(1,535)	(1,846)
Post-employment benefit assets included in other non-current assets	331	262	$ 403
Post-employment benefit obligations	(1,866)	(2,108)	$ (2,105)
Experience gains (losses) included in actuarial gains	(693)	911
Actual return on plan assets	$ 17	$ 1,797
Actual return on plan assets percent	0.20%	8.20%
Benefit obligations
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	$ (26,057)	$ (22,537)
Current service cost	(216)	(214)
Interest on obligations	(920)	(961)
Actuarial gains (losses)	913	(1,221)
Net curtailment (losses) gains	(4)	12
Loss on plan transfer	0	(6)
Benefit payments	1,422	1,401
Employee contributions	(11)	(10)
Acquisition of MTS	0	(2,682)
Plan transfer	0	122
Other	0	39
Post-employment benefit obligations/Fair value of plan assets, December 31	(24,873)	(26,057)
Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	24,244	20,843
Interest on obligations	851	889
Actuarial gains (losses)	(834)	908
Benefit payments	(1,422)	(1,401)
Employer contributions	508	382
Employee contributions	11	10
Acquisition of MTS	0	2,735
Post-employment benefit obligations/Fair value of plan assets, December 31	23,358	24,244
Plan deficit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(1,813)
Post-employment benefit obligations/Fair value of plan assets, December 31	(1,515)	(1,813)
Effect of asset limit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(33)
Post-employment benefit obligations/Fair value of plan assets, December 31	(20)	(33)
DB PENSION PLANS
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(492)
Current service cost	(213)	(208)
Interest on obligations	(23)	(18)
Post-employment benefit obligations/Fair value of plan assets, December 31	(353)	(492)
Post-employment benefit assets included in other non-current assets	331	262
Post-employment benefit obligations	(684)	(754)
DB PENSION PLANS \| Benefit obligations
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(24,404)	(20,853)
Current service cost	(213)	(208)
Interest on obligations	(864)	(896)
Actuarial gains (losses)	750	(1,193)
Net curtailment (losses) gains	(4)	(4)
Loss on plan transfer	0	(6)
Benefit payments	1,342	1,320
Employee contributions	(11)	(10)
Acquisition of MTS	0	(2,677)
Plan transfer	0	122
Other	0	1
Post-employment benefit obligations/Fair value of plan assets, December 31	(23,404)	(24,404)
DB PENSION PLANS \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	23,945	20,563
Interest on obligations	841	878
Actuarial gains (losses)	(817)	896
Benefit payments	(1,342)	(1,320)
Employer contributions	433	305
Employee contributions	11	10
Acquisition of MTS	0	2,735
Post-employment benefit obligations/Fair value of plan assets, December 31	23,071	23,945
DB PENSION PLANS \| Plan deficit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(459)
Post-employment benefit obligations/Fair value of plan assets, December 31	(333)	(459)
DB PENSION PLANS \| Effect of asset limit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(33)
Post-employment benefit obligations/Fair value of plan assets, December 31	(20)	(33)
OPEB PLANS
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(1,354)
Current service cost	(3)	(6)
Interest on obligations	(46)	(54)
Post-employment benefit obligations/Fair value of plan assets, December 31	(1,182)	(1,354)
Post-employment benefit assets included in other non-current assets	0	0
Post-employment benefit obligations	(1,182)	(1,354)
OPEB PLANS \| Benefit obligations
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(1,653)	(1,684)
Current service cost	(3)	(6)
Interest on obligations	(56)	(65)
Actuarial gains (losses)	163	(28)
Net curtailment (losses) gains	0	16
Loss on plan transfer	0	0
Benefit payments	80	81
Employee contributions	0	0
Acquisition of MTS	0	(5)
Plan transfer	0	0
Other	0	38
Post-employment benefit obligations/Fair value of plan assets, December 31	(1,469)	(1,653)
OPEB PLANS \| Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	299	280
Interest on obligations	10	11
Actuarial gains (losses)	(17)	12
Benefit payments	(80)	(81)
Employer contributions	75	77
Employee contributions	0	0
Acquisition of MTS	0	0
Post-employment benefit obligations/Fair value of plan assets, December 31	287	299
OPEB PLANS \| Plan deficit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	(1,354)
Post-employment benefit obligations/Fair value of plan assets, December 31	(1,182)	(1,354)
OPEB PLANS \| Effect of asset limit
Disclosure of net defined benefit liability (asset) [line items]
Post-employment benefit obligations/Fair value of plan assets, January 1	0
Post-employment benefit obligations/Fair value of plan assets, December 31	$ 0	$ 0

Gain (Loss) on Plan Transfer, Net Defined Benefit Liability (Asset)

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Plan Transfer, Net Defined Benefit Liability (Asset)

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Return On Plan Assets, Including Amounts Included In Net Interest On The Net Defined Benefit Liability (Asset)

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No definition available.

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Return On Plan Assets, Including Amounts Included In Net Interest On The Net Defined Benefit Liability (Asset), Percent

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No definition available.

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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No definition available.

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The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from business combinations and disposals. [Refer: Business combinations [member]; Net defined benefit liability (asset)]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

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The amount of non-current net defined benefit asset. [Refer: Net defined benefit asset]

+ References

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The amount of non-current net defined benefit liability. [Refer: Net defined benefit liability]

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The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost and gains (losses) arising from settlements. [Refer: Gains (losses) arising from settlements, net defined benefit liability (asset); Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

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