Redeemable preferred shares - Redeemable Preferred Shares (Details) - Series A Redeemable Convertible Preferred Stock $ / shares in Units, shares in Millions, $ in Millions	May 05, 2018 $ / shares	Feb. 03, 2018 USD ($) $ / shares shares	Apr. 29, 2017 $ / shares	Aug. 04, 2018 USD ($) shares	Jul. 29, 2017 USD ($) shares
Temporary Equity [Line Items]
Conversion ratio	11.1190	10.9409	10.7707
Conversion price (USD per share) \| $ / shares	$ 89.9361	$ 91.4002	$ 92.8445
Conversion of stock, shares issued \| shares		6.8		6.9	6.7
Liquidation preference \| $		$ 632.8		$ 632.8	$ 632.8

The aggregate liquidation preference (or restrictions) of stock classified as temporary equity that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.28(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_TemporaryEquityLiquidationPreference
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant