Accounts receivable - Credit Quality Indicator and Age Analysis (Details) - USD ($) $ in Millions	Feb. 03, 2018	Jul. 29, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross	$ 762.9	$ 1,792.5
Valuation allowance	$ (113.5)	$ (135.0)
Valuation allowance	14.90%	(7.50%)
Financing Receivables, 0 to 120 Days Past Due | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross	$ 703.4
Valuation allowance	(54.0)
Financing Receivables, 121 Or More Days Past Due | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross	59.5
Valuation allowance	$ (59.5)
Current, aged 0 – 30 days | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross		$ 1,394.2
Valuation allowance		(42.8)
Past due, aged 31 – 60 days | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross		264.6
Valuation allowance		(8.6)
Past due, aged 61 – 90 days | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross		52.5
Valuation allowance		(2.4)
Past due, aged more than 90 days | Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross		81.2
Valuation allowance		$ (81.2)