Convertible Senior Notes - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	[1]	Jan. 31, 2017	[1]
Debt Instrument [Line Items]
Total amortization of debt discount and debt issuance costs	$ 21,031	$ 0		$ 0
Convertible Senior Notes
Debt Instrument [Line Items]
Amortization of debt discount	19,611
Amortization of debt issuance costs	1,420
Total amortization of debt discount and debt issuance costs	21,031
Contractual interest expense	584
Total interest expense related to the Notes	$ 21,615
Effective interest rate of the liability component ( as a percent)	5.60%

[1] * As adjusted to reflect the impact of ASU 2016-18 and the full retrospective adoption of