Commitments and Contingencies Diamond Sourcing Activities (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2011	Jan. 31, 2016	Jul. 31, 2015	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan impairment charges		$ 28.3	$ 9.6	$ 37.9	$ 0.0	$ 0.0
Diamond Purchase Commitments Amount Committed				$ 100.0
Koidu Limited [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates	$ 50.0
Loans Receivable Fixed Rate		6.75%		6.75%
Loans Receivable, interest on deferred principal		2.00%		2.00%
Loans and Leases Receivable, Gross		$ 43.8		$ 43.8
Loan impairment charges		28.3	$ 9.6
Loans and Leases Receivable, Net Amount		$ 5.9		$ 5.9
Other diamond mining and exploration companies [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates						$ 3.1
London Interbank Offered Rate (LIBOR) [Member] | Koidu Limited [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Basis Spread on Variable Rate		3.50%		3.50%