Income Taxes - Income Taxes Components Of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Jan. 31, 2018	Jan. 31, 2017
Income Tax Disclosure [Abstract]
Stock-based compensation	$ 26.7	$ 37.6
Research and development tax credit carryforwards	170.3	136.7
Foreign tax credit carryforwards	162.2	127.3
Accrued compensation and benefits	25.9	39.5
Other accruals not currently deductible for tax	22.9	18.7
Purchased technology and capitalized software	43.4	76.9
Fixed assets	16.5	24.3
Tax loss carryforwards	85.7	173.6
Deferred revenue	120.3	128.3
Other	32.4	27.6
Total deferred tax assets	706.3	790.5
Less: valuation allowance	(634.2)	(748.0)
Net deferred tax assets	72.1	42.5
Indefinite lived intangibles	(57.0)	(70.1)
Total deferred tax liabilities	(57.0)	(70.1)
Net deferred tax assets	$ 15.1
Net deferred tax liabilities		$ (27.6)