| Summary of significant accounting policies (Tables) | 6 Months Ended | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Jul. 29, 2017 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Accounting Policies [Abstract] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Black-Scholes Valuation Model Weighted-Average Assumptions | The Company estimated the grant date fair value of stock options using a Black-Scholes valuation model using the following weighted-average assumptions for the periods indicated: 
 
 
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- References No definition available. 
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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