Fair value measurements - Schedule of Fair Value Measurements, Nonrecurring (Details) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Millions	Oct. 28, 2017	Jul. 29, 2017	Apr. 29, 2017	Oct. 29, 2016	Jul. 30, 2016	Apr. 30, 2016
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used	$ 0	$ 1	$ 0	$ 2	$ 0	$ 0
Total assets	1	1	0	2	0	0
Carrying Value Prior to Impairment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used	0	3	0	4	1	0
Total assets	3	3	0	5	1	0
Accumulated Impairment Losses [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used	0	2	0	2	1	0
Total assets	$ 2	$ 2	$ 0	$ 3	$ 1	$ 0