Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net revenue	$ 553.8	$ 515.3	$ 501.8	$ 485.7	$ 478.8	$ 489.6	$ 550.7	$ 511.9	$ 2,056.6	$ 2,031.0	$ 2,504.1
Gross profit	480.7	437.8	427.2	407.5	395.9	408.1	465.6	419.5	1,753.2	1,689.1	2,133.4
Loss from operations	(181.9)	(100.0)	(107.6)	(119.6)	(167.1)	(119.9)	(62.9)	(149.7)	(509.1)	(499.6)	1.3
Provision for income taxes	24.8	(8.6)	(17.6)	(8.2)	(5.2)	(13.5)	(25.2)	(14.4)	(9.6)	(58.3)	(310.2)
Net loss	$ (173.5)	$ (119.8)	$ (144.0)	$ (129.6)	$ (173.4)	$ (142.8)	$ (98.2)	$ (167.7)	$ (566.9)	$ (582.1)	$ (330.5)
Basic net loss per share (usd per share)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.78)	$ (0.64)	$ (0.44)	$ (0.75)	$ (2.58)	$ (2.61)	$ (1.46)
Diluted net loss per share (usd per share)	$ (0.79)	$ (0.55)	$ (0.66)	$ (0.59)	$ (0.78)	$ (0.64)	$ (0.44)	$ (0.75)	$ (2.58)	$ (2.61)	$ (1.46)
Loss from operations includes the following items:
Stock-based compensation expense	$ 62.1	$ 65.1	$ 58.8	$ 59.0	$ 59.3	$ 56.6	$ 54.3	$ 51.6	$ 245.0	$ 221.8
Amortization of acquisition related intangibles	8.6	8.7	8.9	10.4	17.3	17.2	18.5	18.8	36.6	71.8
CEO transition costs	(0.2)	0.0	10.6	11.0					21.4
Restructuring charges and other facility exit costs, net	$ 93.9	$ 0.0	$ 0.5	$ (0.3)	$ 9.0	$ 3.2	$ 16.0	$ 52.3	$ 94.1	$ 80.5	$ 0.0