Investments - Summary of Available-for-sale Securities (Details) (USD $) In Millions, unless otherwise specified	May 02, 2015	Jan. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 23.1	$ 25.2
Unrealized Gain (Loss)	(0.1)	0
Fair Value	23.0	25.2
US Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Cost	9.1	9.7
Unrealized Gain (Loss)	(0.1)	(0.1)
Fair Value	9.0	9.6
US government agency securities
Schedule of Available-for-sale Securities [Line Items]
Cost	0.6	1.4
Unrealized Gain (Loss)	(0.1)	0
Fair Value	0.5	1.4
Corporate bonds and notes
Schedule of Available-for-sale Securities [Line Items]
Cost	10.0	10.6
Unrealized Gain (Loss)	0.1	0.2
Fair Value	10.1	10.8
Corporate equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost	3.4	3.5
Unrealized Gain (Loss)	0	(0.1)
Fair Value	$ 3.4	$ 3.4