Summary of Accounting Policies - Impact of Adoption of ASC 606 (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS:
Unbilled receivables	$ 169.4	$ 146.4	$ 143.6
Other current assets	80.0	72.5	73.5
Other assets	101.1	91.5	88.3
LIABILITIES:
Deferred revenue		552.9
Deferred taxes	931.1	834.2	829.6
Other liabilities	191.5	238.8	239.2
STOCKHOLDERS’ EQUITY:
Retained earnings	$ 6,247.7	5,478.9	5,464.6
Calculated under Revenue Guidance in Effect before ASC 606
ASSETS:
Unbilled receivables			143.6
Other current assets			73.5
Other assets			88.3
LIABILITIES:
Deferred revenue			566.4
Deferred taxes			829.6
Other liabilities			239.2
STOCKHOLDERS’ EQUITY:
Retained earnings			$ 5,464.6
Accounting Standards Update 2014-09 (ASC 606) | Impact of ASC 606 Adoption
ASSETS:
Unbilled receivables		2.8
Other current assets		(1.0)
Other assets		3.2
LIABILITIES:
Deferred revenue		(13.5)
Deferred taxes		4.6
Other liabilities		(0.4)
STOCKHOLDERS’ EQUITY:
Retained earnings		$ 14.3