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Basis of Financial Statements - Recent Developments (Details) $ / shares in Units, $ in Millions							1 Months Ended		3 Months Ended				6 Months Ended
	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 ($)	Jun. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($)	Jul. 26, 2020 USD ($)	Jul. 16, 2020 USD ($)	Jul. 01, 2020	Jun. 26, 2020 $ / shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares							12,650,000
Common stock outstanding (in shares) \| shares							91,656,245	79,516,833		91,656,245			91,656,245
Realized and other gains and losses, net										$ 578.1		$ 75.1	$ 1,493.2	$ 76.7
Investment in partnership							$ 799.7	$ 836.5		799.7			799.7
Prepaid expenses and other current assets							49.9	64.4		49.9			49.9
Equity in (losses) earnings of unconsolidated affiliates										$ 57.5		(21.1)	4.8	(42.5)
Proceeds from equity offering, net of offering expenses							$ 455.0						455.0	0.0
CoreLogic
Subsidiary, Sale of Stock [Line Items]
Acquisition, share price (in usd per share) \| $ / shares																		$ 65.00
Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Additional investment													$ 0.0	526.2
Outstanding voting equity							24.30%			24.30%			24.30%
Investment in partnership							$ 344.0	385.9		$ 344.0			$ 344.0
Equity in (losses) earnings of unconsolidated affiliates										$ (55.5)		(22.9)	$ (45.4)	(47.2)
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							11.10%			11.10%			11.10%
Owned investment (in shares) \| shares							16,100,000			16,100,000			16,100,000
Equity investment											$ 993.4
Investment in partnership							$ 0.0	309.5		$ 0.0			$ 0.0
Equity in (losses) earnings of unconsolidated affiliates										$ 0.0		1.4	$ 1.5	4.0
Blue Ribbon
Subsidiary, Sale of Stock [Line Items]
Outstanding voting equity						65.40%	65.40%			65.40%			65.40%
Debtor-in-possession financing to be provided						$ 20.0										$ 27.5
Realized and other gains and losses, net						26.5
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%			20.30%
Investment in partnership							$ 121.5	0.0		$ 121.5			$ 121.5
Equity in (losses) earnings of unconsolidated affiliates										$ (3.5)		0.0	(3.5)	0.0
AmeriLife \| Cannae and Other Investors
Subsidiary, Sale of Stock [Line Items]
Additional investment					$ 617.0
Equity Fund
Subsidiary, Sale of Stock [Line Items]
Additional investment								$ 90.9					$ 201.2
Outstanding voting equity							46.60%	49.00%		46.60%			46.60%
Investment in partnership							$ 234.1	$ 46.7		$ 234.1			$ 234.1
Equity in (losses) earnings of unconsolidated affiliates										138.1		$ 0.0	79.3	$ 0.0
CoreLogic
Subsidiary, Sale of Stock [Line Items]
Equity investment							154.6			$ 154.6			154.6
Distribution of shares received (in shares) \| shares										2,300,000
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
Blue Ribbon \| Equity Method Investee
Subsidiary, Sale of Stock [Line Items]
Prepaid expenses and other current assets							15.5			$ 15.5			15.5
Equity in (losses) earnings of unconsolidated affiliates										(11.5)
Blue Ribbon \| Equity Method Investee \| DIP Loan
Subsidiary, Sale of Stock [Line Items]
Prepaid expenses and other current assets							$ 4.0			$ 4.0			4.0
Ceridian
Subsidiary, Sale of Stock [Line Items]
Unrealized gain upon change in accounting													$ 684.9
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares							1,650,000
Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Forward purchase agreement, commitment to purchase equity interests, percentage															20.00%
Forward purchase agreement, commitment to purchase equity interests															$ 290.0
Subsequent Event \| Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Common stock outstanding (in shares) \| shares		76,600,000
Subsequent Event \| Black Knight, Inc. \| Optimal Blue \| Forecast
Subsidiary, Sale of Stock [Line Items]
Payments to acquire businesses									$ 1,800.0
Subsequent Event \| 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
Subsequent Event \| Dun And Bradstreet Corporation \| Senior Notes Due 2027
Subsidiary, Sale of Stock [Line Items]
Interest rate																	10.25%
Subsequent Event \| D & B Private Placement
Subsidiary, Sale of Stock [Line Items]
Additional investment		$ 200.0
Subsequent Event \| Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Outstanding voting equity		18.10%
Subsequent Event \| Blue Ribbon
Subsidiary, Sale of Stock [Line Items]
Debtor-in-possession financing to be provided																$ 27.5
Subsequent Event \| Foley Trasimene Acquisition Corp. II \| Forward Purchase Agreements \| Forecast
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
Subsequent Event \| Foley Trasimene Acquisition Corp. II \| Purchase Redeemable Warrants \| Forecast
Subsidiary, Sale of Stock [Line Items]
Commitment to purchase, shares, warrants (in shares) \| shares	5,000,000
Subsequent Event \| 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
Subsequent Event \| Over-Allotment Option \| Dun And Bradstreet Corporation
Subsidiary, Sale of Stock [Line Items]
Sale of stock (in shares) \| shares		11,745,340