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Fair Value Measurements (Narrative) (Detail)
$ / shares in Units, $ in Millions
12 Months Ended
Jan. 31, 2019
USD ($)
$ / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Closing price of company's common stock, dollars per share | $ / shares $ 181.53
2020 Notes  
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Convertible Senior Notes, if-converted excess of principal amount $ 305
2022 Notes  
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Convertible Senior Notes, if-converted excess of principal amount $ 269