Income Taxes - Components Of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Jan. 31, 2019	Jan. 31, 2018
Income Tax Disclosure [Abstract]
Stock-based compensation	$ 25.9	$ 26.7
Research and development tax credit carryforwards	238.7	170.3
Foreign tax credit carryforwards	198.6	162.2
Accrued compensation and benefits	6.5	25.9
Other accruals not currently deductible for tax	19.0	22.9
Purchased technology and capitalized software	32.6	43.4
Fixed assets	15.0	16.5
Tax loss carryforwards	237.2	85.7
Deferred revenue	49.0	120.3
Other	28.4	32.4
Total deferred tax assets	850.9	706.3
Less: valuation allowance	(797.8)	(634.2)
Net deferred tax assets	53.1	72.1
Indefinite lived intangibles	(67.6)	(57.0)
Total deferred tax liabilities	(67.6)	(57.0)
Net deferred tax liabilities	$ (14.5)
Net deferred tax assets		$ 15.1