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Financial Instruments - Cost and Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016
Cash equivalents:
Amortized Cost	$ 886.0	$ 1,078.0	$ 1,213.1	$ 1,353.0
Available-for-sale
Amortized Cost	10.8
Fair Value	11.7
Short-term trading securities
Convertible debt securities, Amortized Cost	4.6	7.5
Convertible debt securities, Gross Unrealized Gains	1.9	0.5
Convertible debt securities, Gross Unrealized Losses	(2.1)	(0.2)
Convertible debt securities, Fair Value	4.4	7.8
Derivative contract assets, Amortized Cost	1.7	2.0
Derivative contract assets, Gross Unrealized Gains	8.6	7.5
Derivative contract assets, Gross Unrealized Losses	(1.8)	(1.3)
Derivative contract assets, Fair Value	8.5	8.2
Derivative contract liabilities, Amortized Cost	0.0	0.0
Derivative contract liabilities, Gross Unrealized Gains	0.0	0.0
Derivative contract liabilities, Gross Unrealized Losses	(7.4)	(26.6)
Derivative contract liabilities, Fair Value	(7.4)	(26.6)
Amortized Cost	445.2	1,080.2
Gross Unrealized Gains	15.3	17.0
Gross Unrealized Losses	(11.3)	(29.2)
Fair Value	449.2	1,068.0
Level 1
Short-term trading securities
Convertible debt securities, Fair Value	0.0	0.0
Derivative contract assets, Fair Value	0.0	0.0
Derivative contract liabilities, Fair Value	0.0	0.0
Fair Value	345.2	392.1
Level 2
Short-term trading securities
Convertible debt securities, Fair Value	0.0	0.0
Derivative contract assets, Fair Value	7.7	7.2
Derivative contract liabilities, Fair Value	(7.4)	(26.6)
Fair Value	98.8	667.1
Level 3
Short-term trading securities
Convertible debt securities, Fair Value	4.4	7.8
Derivative contract assets, Fair Value	0.8	1.0
Derivative contract liabilities, Fair Value	0.0	0.0
Fair Value	5.2	8.8
Agency bonds
Cash equivalents:
Amortized Cost		5.0
Fair Value		5.0
Agency bonds \| Marketable Securities, Current
Available-for-sale
Amortized Cost		0.0
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		0.0
Agency bonds \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		13.7
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.1)
Fair Value		13.6
Agency bonds \| Level 1
Cash equivalents:
Fair Value		5.0
Agency bonds \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Agency bonds \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		13.6
Agency bonds \| Level 2
Cash equivalents:
Fair Value		0.0
Agency bonds \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Agency bonds \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Agency bonds \| Level 3
Cash equivalents:
Fair Value		0.0
Agency bonds \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Agency bonds \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Certificates of deposit
Cash equivalents:
Amortized Cost	1.0	17.4
Fair Value	1.0	17.4
Certificates of deposit \| Marketable Securities, Current
Available-for-sale
Amortized Cost		0.0
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		0.0
Certificates of deposit \| Level 1
Cash equivalents:
Fair Value	0.0	17.4
Certificates of deposit \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Certificates of deposit \| Level 2
Cash equivalents:
Fair Value	1.0	0.0
Certificates of deposit \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Certificates of deposit \| Level 3
Cash equivalents:
Fair Value	0.0	0.0
Certificates of deposit \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Commercial paper
Cash equivalents:
Amortized Cost	87.9	324.2
Fair Value	87.9	324.2
Commercial paper \| Marketable Securities, Current
Available-for-sale
Amortized Cost		27.5
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		27.5
Commercial paper \| Level 1
Cash equivalents:
Fair Value	0.0	0.0
Commercial paper \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Commercial paper \| Level 2
Cash equivalents:
Fair Value	87.9	324.2
Commercial paper \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		27.5
Commercial paper \| Level 3
Cash equivalents:
Fair Value	0.0	0.0
Commercial paper \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Corporate debt securities
Cash equivalents:
Amortized Cost	5.0	5.0
Fair Value	5.0	5.0
Corporate debt securities \| Marketable Securities, Current
Available-for-sale
Amortized Cost		99.4
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.1)
Fair Value		99.3
Corporate debt securities \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		100.2
Gross Unrealized Gains		0.1
Gross Unrealized Losses		(0.4)
Fair Value		99.9
Corporate debt securities \| Level 1
Cash equivalents:
Fair Value	0.0	5.0
Corporate debt securities \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		99.3
Corporate debt securities \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		99.9
Corporate debt securities \| Level 2
Cash equivalents:
Fair Value	5.0	0.0
Corporate debt securities \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Corporate debt securities \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Corporate debt securities \| Level 3
Cash equivalents:
Fair Value	0.0	0.0
Corporate debt securities \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Corporate debt securities \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Custody cash deposit
Cash equivalents:
Amortized Cost	0.8	5.2
Fair Value	0.8	5.2
Custody cash deposit \| Level 1
Cash equivalents:
Fair Value	0.8	5.2
Custody cash deposit \| Level 2
Cash equivalents:
Fair Value	0.0	0.0
Custody cash deposit \| Level 3
Cash equivalents:
Fair Value	0.0	0.0
Money market funds
Cash equivalents:
Amortized Cost	281.4	278.8
Fair Value	281.4	278.8
Money market funds \| Level 1
Cash equivalents:
Fair Value	281.4	0.0
Money market funds \| Level 2
Cash equivalents:
Fair Value	0.0	278.8
Money market funds \| Level 3
Cash equivalents:
Fair Value	0.0	0.0
Municipal bonds
Cash equivalents:
Amortized Cost		5.0
Fair Value		5.0
Municipal bonds \| Marketable Securities, Current
Available-for-sale
Amortized Cost		0.0
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		0.0
Municipal bonds \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		12.7
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.1)
Fair Value		12.6
Municipal bonds \| Level 1
Cash equivalents:
Fair Value		5.0
Municipal bonds \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Municipal bonds \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		12.6
Municipal bonds \| Level 2
Cash equivalents:
Fair Value		0.0
Municipal bonds \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Municipal bonds \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Municipal bonds \| Level 3
Cash equivalents:
Fair Value		0.0
Municipal bonds \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Municipal bonds \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Sovereign debt
Cash equivalents:
Amortized Cost		2.0
Fair Value		2.0
Sovereign debt \| Marketable Securities, Current
Available-for-sale
Amortized Cost		9.2
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		9.2
Sovereign debt \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		2.8
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		2.8
Sovereign debt \| Level 1
Cash equivalents:
Fair Value		0.0
Sovereign debt \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		7.7
Sovereign debt \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Sovereign debt \| Level 2
Cash equivalents:
Fair Value		2.0
Sovereign debt \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		1.5
Sovereign debt \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		2.8
Sovereign debt \| Level 3
Cash equivalents:
Fair Value		0.0
Sovereign debt \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Sovereign debt \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
U.S. government securities
Cash equivalents:
Amortized Cost		0.0
Fair Value		0.0
U.S. government securities \| Marketable Securities, Current
Available-for-sale
Amortized Cost		37.1
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		37.1
U.S. government securities \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		25.5
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.2)
Fair Value		25.3
U.S. government securities \| Level 1
Cash equivalents:
Fair Value		0.0
U.S. government securities \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		37.1
U.S. government securities \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		25.3
U.S. government securities \| Level 2
Cash equivalents:
Fair Value		0.0
U.S. government securities \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
U.S. government securities \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
U.S. government securities \| Level 3
Cash equivalents:
Fair Value		0.0
U.S. government securities \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
U.S. government securities \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Mutual funds
Short-term trading securities
Amortized Cost	56.6	50.1
Gross Unrealized Gains	3.7	8.9
Gross Unrealized Losses	0.0	0.0
Fair Value	60.3	59.0
Mutual funds \| Level 1
Short-term trading securities
Fair Value	60.3	59.0
Mutual funds \| Level 2
Short-term trading securities
Fair Value	0.0	0.0
Mutual funds \| Level 3
Short-term trading securities
Fair Value	0.0	0.0
Asset backed securities \| Marketable Securities, Current
Available-for-sale
Amortized Cost		13.1
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		13.1
Asset backed securities \| Marketable Securities, Noncurrent
Available-for-sale
Amortized Cost		36.8
Gross Unrealized Gains		0.0
Gross Unrealized Losses		(0.2)
Fair Value		36.6
Asset backed securities \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Asset backed securities \| Level 1 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		0.0
Asset backed securities \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value		13.1
Asset backed securities \| Level 2 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		36.6
Asset backed securities \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value		0.0
Asset backed securities \| Level 3 \| Marketable Securities, Noncurrent
Available-for-sale
Fair Value		$ 0.0
Other \| Marketable Securities, Current
Available-for-sale
Amortized Cost	6.2
Gross Unrealized Gains	1.1
Gross Unrealized Losses	0.0
Fair Value	7.3
Other \| Level 1 \| Marketable Securities, Current
Available-for-sale
Fair Value	2.7
Other \| Level 2 \| Marketable Securities, Current
Available-for-sale
Fair Value	4.6
Other \| Level 3 \| Marketable Securities, Current
Available-for-sale
Fair Value	$ 0.0