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Summary of Significant Accounting Policies - Revenue Performance Obligations (Details)
$ in Millions
6 Months Ended
Jul. 31, 2018
USD ($)
Summary of Significant Accounting Policies  
Remaining performance obligations $ 23
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-05-01  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Percentage of remaining performance obligation to be recognized over the next 12 months 62.00%
Period in which remaining performance obligation is expected to be satisfied 1 year