Income taxes - Reconciliation of Income Taxes at Statutory Federal Income Tax Rate to Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax	$ 68,856	$ 39,318	$ 124,287
Research tax credits	6,120	10,044	7,976
Stock-based compensation	(6,395)	(3,004)	(5,124)
Change in valuation allowance	8,566	(42,450)	2,907
Foreign tax rate differential	(6,384)	(4,945)	0
Legal expenses	0	(4,000)	0
Federal tax rate change	0	0	(132,387)
Global intangible low-taxed income	(3,668)	0	0
Non-deductible compensation	(1,150)	0	0
Change in U.S. tax status of foreign entities	(72,449)	0	0
Other	(2,196)	(381)	262
Total provision for income taxes	$ (8,700)	$ (5,418)	$ (2,079)