Supplemental Financial Data - Computer Software and Goodwill (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 07, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Computer Software
Balance at beginning of period	$ 133.8	$ 379.8	$ 133.8
Acquisitions		0.0	376.0
Additions at cost	5.0	113.1	56.4
Amortization	(6.8)	(70.8)	(48.4)	$ (44.9)
Write-off		(1.0)
Other	0.1	11.6	(4.2)
Balance at end of period	132.1	432.7	379.8	133.8
Goodwill
Balance at beginning of period	770.8	2,840.1	770.8
Acquisitions		10.9	2,842.8
Other	2.7	5.2	(2.7)
Balance at end of period	773.5	2,856.2	2,840.1	770.8
Computer software amortization expense	$ 6.8	$ 70.8	$ 48.4	$ 44.9