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Basis of Financial Statements - Recent Developments (Details) $ / shares in Units, $ in Millions											1 Months Ended		3 Months Ended			9 Months Ended
	Nov. 09, 2020 USD ($) shares	Oct. 30, 2020 shares	Oct. 07, 2020	Sep. 15, 2020 USD ($)	Jul. 31, 2020 USD ($) numberOfShares $ / shares shares	Jul. 06, 2020 USD ($) $ / shares shares	Jun. 05, 2020 USD ($) numberOfShares $ / shares shares	May 08, 2020 USD ($) numberOfShares $ / shares shares	Mar. 18, 2020 USD ($)	Jan. 27, 2020 USD ($)	Jun. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Nov. 17, 2020 director	Oct. 02, 2020	Sep. 14, 2020 $ / shares	Jul. 01, 2020	Jun. 26, 2020 $ / shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares											12,650,000
Additional investment				$ 289.0
Common stock outstanding (in shares) \| shares												79,516,833	91,656,245			91,656,245
Recognized gains													$ (189.6)		$ (92.9)	$ (1,682.8)	$ (169.6)
Investment in partnership												$ 836.5	1,413.2			1,413.2
Prepaid expenses and other current assets												64.4	75.1			75.1
Equity in earnings (losses) from equity investments													(1.2)		(8.3)	3.6	(50.8)
Proceeds from equity offering, net of offering expenses											$ 455.0					455.0	0.0
Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Common stock outstanding (in shares) \| shares						76,600,000
CoreLogic
Subsidiary, Sale of Stock [Line Items]
Acquisition, share price (in usd per share) \| $ / shares																				$ 66.00		$ 65.00
Affiliated Entity \| Corporate Services Agreement \| Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Corporate Services Agreement, term			2 years
Corporate Services Agreement, standard allocation, plus additional percentage			10.00%
Corporate services Agreement, renewal term			1 year
Corporate Services Agreement, notice of termination period			30 days
Blue Ribbon \| Equity Method Investee
Subsidiary, Sale of Stock [Line Items]
Prepaid expenses and other current assets													27.5			27.5
Equity in earnings (losses) from equity investments													5.2			(6.3)
Blue Ribbon \| Equity Method Investee \| Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Outstanding voting equity																			100.00%
Blue Ribbon \| Equity Method Investee \| DIP Loan
Subsidiary, Sale of Stock [Line Items]
Prepaid expenses and other current assets													$ 21.2			21.2
Ceridian
Subsidiary, Sale of Stock [Line Items]
Unrealized gain upon change in accounting																684.9
D & B Private Placement
Subsidiary, Sale of Stock [Line Items]
Additional investment						$ 200.0
Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Additional investment																$ 200.0	526.2
Outstanding voting equity						18.10%							18.10%			18.10%
Investment in partnership												385.9	$ 668.5			$ 668.5
Equity in earnings (losses) from equity investments													$ (3.1)		(21.4)	$ (48.5)	(68.6)
Ceridian
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares																7,600,000
Proceeds from sale of stock																$ 522.0
Outstanding voting equity													10.90%			10.90%
Owned investment (in shares) \| shares													16,100,000			16,100,000
Equity investment											$ 993.4			$ 993.4
Recognized gains																$ 47.1
Investment in partnership												309.5	$ 0.0			0.0
Equity in earnings (losses) from equity investments													$ 0.0		12.7	$ 1.5	16.7
Ceridian \| Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares	2,100,000
Proceeds from sale of stock	$ 198.9
Blue Ribbon
Subsidiary, Sale of Stock [Line Items]
Outstanding voting equity										65.40%
Recognized gains										$ (26.5)
Debtor-in-possession financing to be provided										27.5
Investment in partnership										$ 33.6
Other than temporary impairment of investment														$ 18.6
AmeriLife
Subsidiary, Sale of Stock [Line Items]
Additional investment									$ 125.0
Outstanding voting equity									20.30%				20.30%			20.30%
Investment in partnership												0.0	$ 120.7			$ 120.7
Equity in earnings (losses) from equity investments													$ (0.8)		0.0	(4.3)	0.0
Senator Focused Strategies
Subsidiary, Sale of Stock [Line Items]
Additional investment												$ 90.9				$ 201.2
Outstanding voting equity												49.00%	87.40%			87.40%
Investment in partnership												$ 46.7	$ 235.7			$ 235.7
Equity in earnings (losses) from equity investments													$ 1.6		0.0	80.9	0.0
Distribution of shares received (in shares) \| shares													232,400,000
Senator Focused Strategies \| Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Distribution of shares owned , increased ownership \| shares		2,300,000
CoreLogic
Subsidiary, Sale of Stock [Line Items]
Equity investment													$ 155.6			155.6
Distribution of shares received (in shares) \| shares													2,300,000
CoreLogic \| Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Number of directors to be appointed \| director																		9
Foley Trasimene Acquisition Corp
Subsidiary, Sale of Stock [Line Items]
Number of shares called by each warrant \| numberOfShares								1
Foley Trasimene Acquisition Corp \| Forward Purchase Agreements
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares								15,000,000
Commitment to purchase, share price (in usd per share) \| $ / shares								$ 11.50
Commitment to purchase								$ 150.0
Foley Trasimene Acquisition Corp \| Purchase Redeemable Warrants
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares								5,000,000
Trebia
Subsidiary, Sale of Stock [Line Items]
Number of shares called by each warrant \| numberOfShares							1
Trebia \| Forward Purchase Agreements
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares							7,500,000
Commitment to purchase, share price (in usd per share) \| $ / shares							$ 11.50
Commitment to purchase							$ 75.0
Trebia \| Purchase Redeemable Warrants
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares							2,500,000
Foley Trasimene Acquisition Corp. II \| Forward Purchase Agreements
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares					15,000,000
Number of shares called by each warrant \| numberOfShares					1
Commitment to purchase, share price (in usd per share) \| $ / shares					$ 11.50
Commitment to purchase					$ 150.0
Foley Trasimene Acquisition Corp. II \| Purchase Redeemable Warrants
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares					5,000,000
Optimal Blue
Subsidiary, Sale of Stock [Line Items]
Additional investment																$ 289.0	0.0
Outstanding voting equity													20.00%			20.00%
Investment in partnership												$ 0.0	$ 284.3			$ 284.3
Equity in earnings (losses) from equity investments													(4.7)		$ 0.0	$ (4.7)	$ 0.0
Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Proceeds from sale of stock						$ 2,400.0
Proceeds from issuance of equity						$ 400.0
Dun And Bradstreet Corporation \| Senior Notes Due 2027
Subsidiary, Sale of Stock [Line Items]
Interest rate																					10.25%
Cannae and Other Investors \| AmeriLife
Subsidiary, Sale of Stock [Line Items]
Additional investment									$ 617.0
Black Knight, Inc. \| Optimal Blue
Subsidiary, Sale of Stock [Line Items]
Outstanding voting equity				20.00%
IPO \| Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares						90,047,612
Sale of stock, price per share (in usd per share) \| $ / shares						$ 22.00
Gain on sale of stock by equity investee													117.0
Gain on sale of stock by equity investee, tax expense													$ 2.3
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares											1,650,000
Over-Allotment Option \| Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares						11,745,340