Income Taxes - Income Taxes Differences between the U.S. statutory rate and the aggregate income tax provision (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	[1]	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2015
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at U.S. Federal statutory rate										$ (177.0)	$ (7.1)	$ 29.0
State income tax benefit, net of the U.S. Federal benefit										(17.3)	(7.6)	(4.0)
Foreign income taxed at rates different from the U.S. statutory rate										22.3	(29.4)	(40.0)
U.S. valuation allowance										233.0	345.0	2.9
Increase in attributes due to ASU 2016-9 adoption										(119.4)	0.0	0.0
Change in valuation allowance from ASU 2016-9 adoption										119.4	0.0	0.0
Tax effect of non-deductible stock-based compensation										18.8	19.3	15.7
Stock compensation windfall shortfall										(23.0)	0.0	0.0
Research and development tax credit benefit										(10.3)	(9.4)	(7.2)
Closure of income tax audits and changes in uncertain tax positions										8.2	(4.7)	(0.7)
Tax effect of officer compensation in excess of $1.0 million										2.2	1.4	2.4
Non-deductible expenses										2.0	2.6	2.2
Other										(0.6)	0.1	0.9
Income tax expense	$ 5.2	$ 13.5	$ 25.2		$ 14.4	$ 16.7	$ 21.3	$ 269.5	$ 2.7	$ 58.3	$ 310.2	$ 1.2

[1] Certain first quarter fiscal 2017 balances have been revised to reflect the adoption of ASU 2016-09. As the adoption occurred in the second quarter of fiscal 2017, we are required to reflect any adjustments as of February 1, 2016, the beginning of the annual period that includes the interim period of adoption, and are required to revise our reported quarterly results for the first quarter. The revision resulted in a $5.3 million decrease of stock-based compensation expense from the previously reported expense. See Note 1, "Business and Summary of Significant Accounting Policies" for further discussion on the adoption.