Subsequent events (Details) - shares shares in Millions		12 Months Ended
Jan. 29, 2019	Dec. 31, 2018
Subsequent events
Subsequent event, description		On January 29, 2019, the Board of Directors of the Company approved a new NCIB, which allows for the repurchase of up to 22 million common shares between February 1, 2019 and January 31, 2020, at prevailing market prices plus brokerage fees, or such other prices may be permitted by the Toronto Stock Exchange.
Subsequent Event [Member]
Subsequent events
Subsequent event, date	Jan. 29, 2019
Common shares approved for repurchase (in shares)	22

The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

No definition available.

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- Definition

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 855
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Definition

Date the event occurred after the balance sheet date but before financial statements are issues or available to be issued, in CCYY-MM-DD format.

+ References

+ Details

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- Details

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