Immaterial Correction of Prior Period Financial Statements - Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 123.4			$ 80.2	$ 341.8	$ 213.5
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Deferred tax assets					4.4	11.5
Deferred revenue					282.6	255.7
Net cash provided by operating activities					$ 787.2	617.6
As Previously Reported [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income				79.8		211.3
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Deferred tax assets						10.6
Deferred revenue						258.8
Net cash provided by operating activities						617.6
Corrections [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		$ 1.7	$ 0.6	$ 0.4		2.2
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Deferred tax assets						0.9
Deferred revenue						(3.1)
Net cash provided by operating activities						$ 0.0