Stock-Based Compensation - Stock options - other details (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018	Jan. 31, 2019
Stock Based Compensation
Options unvested (in shares)	1,107,201		1,107,201
Aggregate intrinsic value of options vested and exercisable	$ 1,034.0		$ 1,034.0		$ 181.1
Weighted-average remaining contractual term of options vested and exercisable (in years)			6 years 6 months		7 years 1 month 6 days
Weighted-average grant date fair value of options granted (in dollars per share)	$ 13.45	$ 5.36	$ 9.51	$ 5.36
Total intrinsic value of options exercised	$ 80.9	$ 6.6	$ 89.7	$ 7.6
Aggregate intrinsic value of stock options outstanding	1,797.2		$ 1,797.2		$ 286.1
Weighted-average remaining contractual term of stock options outstanding (in years)			7 years 6 months		7 years 10 months 24 days
Total unrecognized stock-based compensation expenses related to unvested options	$ 44.5		$ 44.5		$ 45.8
Expected to be amortized over weighted-average vesting period (in years)			2 years 6 months		3 years 4 months 24 days