Financial Instruments (Cost and Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Jul. 31, 2018	Jan. 31, 2018	Jul. 31, 2017	Jan. 31, 2017
Cash equivalents
Amortized Cost	$ 895.4	$ 1,078.0	$ 1,174.1	$ 1,213.1
Marketable securities:
Amortized Cost	349.3
Short-term trading securities
Derivative contract assets, Amortized Cost	1.7	2.0
Derivative contract assets, Gross unrealized gains	12.8	7.5
Derivative contract assets, Gross unrealized losses	(1.6)	(1.3)
Derivative contract assets	12.9	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	(6.9)	(26.6)
Derivative contract liabilities	(6.9)	(26.6)
Amortized Cost	837.0	1,080.2
Gross unrealized gains	22.4	17.0
Gross unrealized losses	(12.2)	(29.2)
Fair Value	847.2	1,068.0
Other Assets
Short-term trading securities
Convertible debt securities, Amortized Cost	7.5	7.5
Convertible debt securities, Gross unrealized gains	1.4	0.5
Convertible debt securities, Gross unrealized losses	(2.3)	(0.2)
Convertible debt securities, Fair Value	6.6	7.8
Level 1
Short-term trading securities
Derivative contract assets	0.0	0.0
Derivative contract liabilities	0.0	0.0
Fair Value	186.6	392.1
Level 1 | Other Assets
Short-term trading securities
Convertible debt securities, Fair Value	0.0	0.0
Level 2
Short-term trading securities
Derivative contract assets	12.0	7.2
Derivative contract liabilities	(6.9)	(26.6)
Fair Value	653.1	667.1
Level 2 | Other Assets
Short-term trading securities
Convertible debt securities, Fair Value	0.0	0.0
Level 3
Short-term trading securities
Derivative contract assets	0.9	1.0
Derivative contract liabilities	0.0	0.0
Fair Value	7.5	8.8
Level 3 | Other Assets
Short-term trading securities
Convertible debt securities, Fair Value	6.6	7.8
Agency bonds
Cash equivalents
Amortized Cost	5.0	5.0
Fair Value	5.0	5.0
Agency bonds | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	12.5
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	12.5
Agency bonds | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	12.5	13.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	12.4	13.6
Agency bonds | Level 1
Cash equivalents
Fair Value	0.0	5.0
Agency bonds | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Agency bonds | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	13.6
Agency bonds | Level 2
Cash equivalents
Fair Value	5.0	0.0
Agency bonds | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	12.5
Agency bonds | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	12.4	0.0
Agency bonds | Level 3
Cash equivalents
Fair Value	0.0	0.0
Agency bonds | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Agency bonds | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Asset backed securities | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	12.1	13.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	12.1	13.1
Asset backed securities | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	27.1	36.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.3)	(0.2)
Fair Value	26.8	36.6
Asset backed securities | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Asset backed securities | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Asset backed securities | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	12.1	13.1
Asset backed securities | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	26.8	36.6
Asset backed securities | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Asset backed securities | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Certificates of deposit
Cash equivalents
Amortized Cost	11.5	17.4
Fair Value	11.5	17.4
Certificates of deposit | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	7.7
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	7.7
Certificates of deposit | Level 1
Cash equivalents
Fair Value	0.0	17.4
Certificates of deposit | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Certificates of deposit | Level 2
Cash equivalents
Fair Value	11.5	0.0
Certificates of deposit | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	7.7
Certificates of deposit | Level 3
Cash equivalents
Fair Value	0.0	0.0
Certificates of deposit | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Commercial paper
Cash equivalents
Amortized Cost	269.5	324.2
Fair Value	269.5	324.2
Commercial paper | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	62.7	27.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	62.7	27.5
Commercial paper | Level 1
Cash equivalents
Fair Value	0.0	0.0
Commercial paper | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Commercial paper | Level 2
Cash equivalents
Fair Value	269.5	324.2
Commercial paper | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	62.7	27.5
Commercial paper | Level 3
Cash equivalents
Fair Value	0.0	0.0
Commercial paper | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Corporate debt securities
Cash equivalents
Amortized Cost		5.0
Fair Value		5.0
Corporate debt securities | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	64.9	99.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.2)	(0.1)
Fair Value	64.7	99.3
Corporate debt securities | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	74.0	100.2
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.5)	(0.4)
Fair Value	73.6	99.9
Corporate debt securities | Level 1
Cash equivalents
Fair Value		5.0
Corporate debt securities | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	99.3
Corporate debt securities | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	99.9
Corporate debt securities | Level 2
Cash equivalents
Fair Value		0.0
Corporate debt securities | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	64.7	0.0
Corporate debt securities | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	73.6	0.0
Corporate debt securities | Level 3
Cash equivalents
Fair Value		0.0
Corporate debt securities | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Corporate debt securities | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Custody cash deposit
Cash equivalents
Amortized Cost	1.7	5.2
Fair Value	1.7	5.2
Custody cash deposit | Level 1
Cash equivalents
Fair Value	1.7	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
Municipal bonds
Cash equivalents
Amortized Cost	5.0	5.0
Fair Value	5.0	5.0
Municipal bonds | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	5.0	9.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	5.0	9.2
Municipal bonds | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	7.7	12.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	7.6	12.6
Municipal bonds | Level 1
Cash equivalents
Fair Value	0.0	5.0
Municipal bonds | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	7.7
Municipal bonds | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	12.6
Municipal bonds | Level 2
Cash equivalents
Fair Value	5.0	0.0
Municipal bonds | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	5.0	1.5
Municipal bonds | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	7.6	0.0
Municipal bonds | Level 3
Cash equivalents
Fair Value	0.0	0.0
Municipal bonds | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
Municipal bonds | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Money market funds
Cash equivalents
Amortized Cost	122.9	278.8
Fair Value	122.9	278.8
Money market funds | Level 1
Cash equivalents
Fair Value	122.9	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
Sovereign debt
Cash equivalents
Amortized Cost	5.0	2.0
Fair Value	5.0	2.0
Sovereign debt | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	6.5
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	6.5
Sovereign debt | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	2.7	2.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	2.7	2.8
Sovereign debt | Level 1
Cash equivalents
Fair Value	0.0	0.0
Sovereign debt | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Sovereign debt | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Sovereign debt | Level 2
Cash equivalents
Fair Value	5.0	2.0
Sovereign debt | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	6.5
Sovereign debt | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	2.7	2.8
Sovereign debt | Level 3
Cash equivalents
Fair Value	0.0	0.0
Sovereign debt | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0
Sovereign debt | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
U.S. government securities
Cash equivalents
Amortized Cost	11.5
Fair Value	11.5
U.S. government securities | Marketable Securities, Short-term
Marketable securities:
Amortized Cost	41.4	37.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.2)	0.0
Fair Value	41.2	37.1
U.S. government securities | Marketable Securities, Long-term
Marketable securities:
Amortized Cost	5.0	25.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.2)
Fair Value	5.0	25.3
U.S. government securities | Level 1
Cash equivalents
Fair Value	0.0
U.S. government securities | Level 1 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	37.1
U.S. government securities | Level 1 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	25.3
U.S. government securities | Level 2
Cash equivalents
Fair Value	11.5
U.S. government securities | Level 2 | Marketable Securities, Short-term
Marketable securities:
Fair Value	41.2	0.0
U.S. government securities | Level 2 | Marketable Securities, Long-term
Marketable securities:
Fair Value	5.0	0.0
U.S. government securities | Level 3
Cash equivalents
Fair Value	0.0
U.S. government securities | Level 3 | Marketable Securities, Short-term
Marketable securities:
Fair Value	0.0	0.0
U.S. government securities | Level 3 | Marketable Securities, Long-term
Marketable securities:
Fair Value	0.0	0.0
Mutual funds
Short-term trading securities
Amortized Cost	53.9	50.1
Gross Unrealized Gains	8.1	8.9
Gross Unrealized Losses	0.0	0.0
Fair Value	62.0	59.0
Mutual funds | Level 1
Short-term trading securities
Fair Value	62.0	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