Income taxes - Significant components of tax expense (income) (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Current taxes
Current taxes	$ (758)	$ (850)
Uncertain tax positions	(9)	(14)
Change in estimate relating to prior periods	40	14
Other	0	(1)
Deferred taxes
Deferred taxes relating to the origination and reversal of temporary differences	(41)	(299)
Change in estimate relating to prior periods	11	32
Recognition and utilization of loss carryforwards	(304)	(1)
Effect of change in provincial corporate tax rate	(3)	4
Resolution of uncertain tax positions	25	5
Total income taxes	$ (1,039)	$ (1,110)

Adjustments For Current Tax Expense (Income) Of Uncertain Tax Positions

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Deferred Tax Expense (Income) Relating To Recognition and Utilization of Loss Carryforwards

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Other Components Of Current Tax Expense (Income)

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Adjustments of tax expense (income) recognised in the period for current tax of prior periods.

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Adjustments of tax expense (income) recognised in the period for the deferred tax of prior periods.

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The amount of income taxes payable (recoverable) in respect of the taxable profit (tax loss) for a period.

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The amount of deferred tax expense or income relating to the creation or reversal of temporary differences. [Refer: Temporary differences [member]; Deferred tax expense (income)]

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The amount of deferred tax expense or income relating to tax rate changes or the imposition of new taxes. [Refer: Deferred tax expense (income)]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax that relate to continuing operations. [Refer: Continuing operations [member]; Current tax expense (income); Deferred tax expense (income)]

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The amount of benefit arising from a previously unrecognised tax loss, tax credit or temporary difference of a prior period that is used to reduce deferred tax expense. [Refer: Deferred tax expense (income); Temporary differences [member]; Unused tax losses [member]; Unused tax credits [member]]

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