Income Taxes - Components Of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Jan. 31, 2020	Jan. 31, 2019
Income Tax Disclosure [Abstract]
Stock-based compensation	$ 32.8	$ 25.9
Research and development tax credit carryforwards	263.4	238.7
Foreign tax credit carryforwards	253.9	198.6
Accrued compensation and benefits	3.4	6.5
Other accruals not currently deductible for tax	28.4	19.0
Purchased technology and capitalized software	37.7	32.6
Fixed assets	11.6	15.0
Lease liability	106.4	0.0
Tax loss carryforwards	241.2	237.2
Deferred revenue	29.2	49.0
Other	28.0	28.4
Total deferred tax assets	1,036.0	850.9
Less: valuation allowance	(883.4)	(797.8)
Net deferred tax assets	152.6	53.1
Indefinite lived intangibles	76.5	67.6
Right-of-use assets	(101.3)	0.0
Unremitted earnings of foreign subsidiaries	(0.9)	0.0
Total deferred tax liabilities	(178.7)	(67.6)
Net deferred tax liabilities	$ (26.1)	$ (14.5)