Significant Accounting Policies - Revenue Performance Obligations (Details) $ in Millions	12 Months Ended
	Jan. 31, 2019 USD ($)
Significant Accounting Policies
Revenue, Remaining Performance Obligation	$ 27
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-02-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Percentage of remaining performance obligation to be recognized over the next 12 months	65.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	12 months