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Commitments and Contingencies Diamond Sourcing Activities (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2011
Apr. 30, 2017
Jan. 31, 2017
Jan. 31, 2016
Jul. 31, 2015
Jan. 31, 2017
Jan. 31, 2016
Jan. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]                
Loan impairment charges     $ 12.6 $ 28.3 $ 9.6 $ 12.6 $ 37.9 $ 0.0
Diamond Purchase Commitments Amount Committed           $ 60.0    
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           4.2 37.9  
Loans and Leases Receivable, Net Amount     $ 1.7 $ 5.9   $ 1.7 $ 5.9  
Koidu Limited [Member] | Subsequent Event [Member]                
Accounts, Notes, Loans and Financing Receivable [Line Items]                
Proceeds from Sale of Finance Receivables   $ 1.7            
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%