Financial Instruments - Contractual Maturities of Types of Securities (Details) $ in Millions	Jan. 31, 2018 USD ($)
Cost
Due within in 1 year	$ 193.8
Due in 1 year through 5 years	186.9
Due in 5 years through 10 years	3.7
Due after 10 years	1.1
Total	385.5
Fair Value
Due within in 1 year	194.0
Due in 1 year through 5 years	186.0
Due in 5 years through 10 years	3.7
Due after 10 years	1.1
Total	$ 384.8