Acquisitions (Summary of Fair Value of Assets Acquired and Liabilities Assumed by Major Class) (Details) - USD ($) $ in Millions	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019
Business Acquisition [Line Items]
Goodwill	$ 2,445.0	$ 2,450.8
Fiscal 2019 Acquisitions
Business Acquisition [Line Items]
Goodwill	868.0
Deferred revenue (current and non-current)	(30.0)
Net tangible assets	25.0
Total	1,124.4
Fiscal 2019 Acquisitions | Developed technologies
Business Acquisition [Line Items]
Finite-lived intangibles	94.9
Fiscal 2019 Acquisitions | Customer relationships and other non-current intangible assets
Business Acquisition [Line Items]
Finite-lived intangibles	136.9
Fiscal 2019 Acquisitions | Trade name
Business Acquisition [Line Items]
Finite-lived intangibles	29.6
Assemble Systems Inc
Business Acquisition [Line Items]
Goodwill	71.8
Deferred revenue (current and non-current)	1.7
Net tangible assets	4.3
Total	93.6
Goodwill, measurement period adjustment	0.2
Net tangible assets, measurement period adjustment	(0.2)
Assemble Systems Inc | Developed technologies
Business Acquisition [Line Items]
Finite-lived intangibles	4.4
Assemble Systems Inc | Customer relationships and other non-current intangible assets
Business Acquisition [Line Items]
Finite-lived intangibles	12.0
Assemble Systems Inc | Trade name
Business Acquisition [Line Items]
Finite-lived intangibles	2.8
PlanGrid
Business Acquisition [Line Items]
Goodwill	589.5
Deferred revenue (current and non-current)	25.5
Net tangible assets	17.6
Total	777.6
Goodwill, measurement period adjustment	0.8
PlanGrid | Developed technologies
Business Acquisition [Line Items]
Finite-lived intangibles	78.0
PlanGrid | Customer relationships and other non-current intangible assets
Business Acquisition [Line Items]
Finite-lived intangibles	98.0
PlanGrid | Trade name
Business Acquisition [Line Items]
Finite-lived intangibles	20.0
BuildingConnected, Inc.
Business Acquisition [Line Items]
Goodwill	206.7
Deferred revenue (current and non-current)	2.8
Net tangible assets	3.1
Total	253.2
Goodwill, measurement period adjustment	0.4
BuildingConnected, Inc. | Developed technologies
Business Acquisition [Line Items]
Finite-lived intangibles	12.5
BuildingConnected, Inc. | Customer relationships and other non-current intangible assets
Business Acquisition [Line Items]
Finite-lived intangibles	26.9
BuildingConnected, Inc. | Trade name
Business Acquisition [Line Items]
Finite-lived intangibles	$ 6.8