Summary of Significant Accounting Policies Receivables and Financing Arrangements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan impairment charges	$ 12.6	$ 3.0	$ 12.6	$ 37.9
Consumer Credit Card Financing Receivables	$ 71.9	$ 67.1	$ 71.9
Consumer Credit Card Financing Receivables Percentage Current	97.00%	95.00%	97.00%
Consumer Credit Card Financing Receivable Allowance For Credit Losses	$ 1.1	$ 1.3	$ 1.1
Financing Receivables Gross Current	4.6		4.6
Financing Receivables Gross Non Current	$ 4.4		4.4
Koidu Limited [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		43.8
Loan impairment charges		4.2	$ 37.9
Loans and Leases Receivable, Net Amount		1.7
Proceeds from Sale of Finance Receivables		1.7
Other Diamond Mining and Exploration Company [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan impairment charges		$ 8.4