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Debt - Senior Notes (Details)
$ in Millions, ¥ in Billions
12 Months Ended
Jan. 31, 2019
JPY (¥)
day
Jan. 31, 2019
USD ($)
Aug. 31, 2016
JPY (¥)
2016 0.78% Senior Notes, due August 2026      
Debt Instrument [Line Items]      
Debt, face amount ¥ 10.0 $ 91.8 ¥ 10.0
Interest rate, stated percentage 0.78% 0.78%  
Principal amount redeemed (as a percent) 100.00%    
2010 1.72% Notes, due September 2016      
Debt Instrument [Line Items]      
Debt, face amount | ¥ ¥ 10.0    
Interest rate, stated percentage 1.72% 1.72%  
Minimum | 2016 0.78% Senior Notes, due August 2026      
Debt Instrument [Line Items]      
Days notice to redeem notes (in days) 30    
Maximum | 2016 0.78% Senior Notes, due August 2026      
Debt Instrument [Line Items]      
Days notice to redeem notes (in days) 60