Goodwill and Other Intangibles - Computer Software and Goodwill (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 08, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Computer Software
Balance at beginning of period	$ 0.0	$ 391.8	$ 381.1	$ 379.8	$ 372.1	$ 372.0	$ 0.0
Acquisitions		0.0		0.0	0.0		376.0
Additions at cost	5.0	35.3	28.3	17.4	19.0	17.0	4.2
Amortization	(6.8)	(18.2)	(16.8)	(15.9)	(12.2)	(13.4)	(8.6)
Other	0.1	12.9	(0.7)		(3.6)	(3.5)	0.4
Write-off		(0.2)	(0.1)	(0.2)
Balance at end of period	132.1	421.6	391.8	381.1	375.3	372.1	372.0
Goodwill
Balance at beginning of period	0.0	2,848.0	2,850.8	2,840.1	2,792.6	2,797.6	0.0
Acquisitions		0.2		10.7	180.9		2,797.6
Other	2.7	5.7	(2.8)		(6.1)	(5.0)	0.0
Balance at end of period	773.5	$ 2,853.9	$ 2,848.0	$ 2,850.8	$ 2,967.4	$ 2,792.6	2,797.6
Dun & Bradstreet Corp
Computer Software
Balance at beginning of period	133.8						133.8
Goodwill
Balance at beginning of period	$ 770.8						$ 770.8