Significant Accounting Policies - Textuals (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Customer	Dec. 31, 2016 USD ($) Customer	Dec. 31, 2015 USD ($) Customer	Jan. 01, 2017 USD ($)
Significant Accounting Policies [Line Items]
Maturity of highly liquid investments	3 months
Contractual period	1 year
Warranty period for hardware products (in years)	1 year
Warranty period for software (in days)	90 days
Advertising expense	$ 19.9	$ 15.8	$ 20.2
More than likely percentage of being realized upon settlement, tax benefit	50.00%
Liability for unrecognized tax benefits as current, period	1 year
Number of customers accounting for more than 10% of revenues | Customer	0	0	0
Deferred revenue	$ 1,539.3	$ 1,481.1
Net Cash Provided by (Used in) Operating Activities	$ 1,260.1	1,107.2	$ 899.5
Minimum
Significant Accounting Policies [Line Items]
Contractual support period	1 year
Maximum
Significant Accounting Policies [Line Items]
Contractual support period	3 years
Computer, Equipment and Software | Minimum
Significant Accounting Policies [Line Items]
Useful life	1 year 6 months
Computer, Equipment and Software | Maximum
Significant Accounting Policies [Line Items]
Useful life	7 years
Furniture and fixtures | Minimum
Significant Accounting Policies [Line Items]
Useful life	5 years
Furniture and fixtures | Maximum
Significant Accounting Policies [Line Items]
Useful life	7 years
Building and building improvements | Minimum
Significant Accounting Policies [Line Items]
Useful life	7 years
Building and building improvements | Maximum
Significant Accounting Policies [Line Items]
Useful life	40 years
Land Improvements | Minimum
Significant Accounting Policies [Line Items]
Useful life	10 years
Land Improvements | Maximum
Significant Accounting Policies [Line Items]
Useful life	40 years
Leasehold Improvements
Significant Accounting Policies [Line Items]
Useful life	10 years
Accounting Standards Update 2016-09, Excess Tax Benefit Component
Significant Accounting Policies [Line Items]
Net Cash Provided by (Used in) Operating Activities		$ 6.7	$ 12.3
Accounting Standards Update 2016-09, Excess Tax Benefit Component | Accumulated Deficit
Significant Accounting Policies [Line Items]
Cumulative adjustment for share-based compensation expense upon adoption of Accounting Standards Update (ASU) 2016-09, net of tax				$ 9.0