Related Parties - Narrative (Details) $ / shares in Units, $ in Millions					1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 06, 2020 USD ($) shares	Jun. 30, 2020 USD ($) $ / shares shares	Jan. 01, 2020	Feb. 08, 2019 USD ($)	Jun. 30, 2020 USD ($)	Feb. 07, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) profitInterestUnit	Dec. 31, 2019 USD ($)	Aug. 31, 2019
Related Party Transaction [Line Items]
Stock options granted during period (shares) | shares									8,000,000
Total compensation expense for stock options granted						$ 11.7	$ 9.7	$ 3.6	$ 38.6	$ 7.9
Stock issued (shares) | shares	108,506,312
Exercise price of stock options granted during period (USD per share) | $ / shares									$ 22.00
Gross proceeds from sale of stock	$ 2,381.0
Stock Options
Related Party Transaction [Line Items]
Total compensation expense for stock options granted						$ 0.0	1.7	$ 0.0	$ 21.7	0.0
Private Placement
Related Party Transaction [Line Items]
Stock issued (shares) | shares	18,458,700
Issuance price per share relative to IPO price per share (as a percent)	98.50%
Stock Issuance Costs | Star Parent, L.P.
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 30.0
Board of Directors Chairman & Director | Stock Options
Related Party Transaction [Line Items]
Total compensation expense for stock options granted		$ 20.0
Board of Directors Chairman & Director | Class B Profit Interest Units
Related Party Transaction [Line Items]
Fees/ expenses with related party included in "Selling and Administrative Expenses"										$ 17.3
Number of profit interest units granted (shares) | profitInterestUnit										6,817.7428
Board of Directors Chairman & Director | Class C Profit Interest Units
Related Party Transaction [Line Items]
Fees/ expenses with related party included in "Selling and Administrative Expenses"										$ 37.9
Number of profit interest units granted (shares) | profitInterestUnit										15,867.8087
Board of Directors Chairman
Related Party Transaction [Line Items]
Stock options granted during period (shares) | shares		2,080,000
Board of Directors Chairman | Trasimene Capital Management, LLC
Related Party Transaction [Line Items]
Term of service agreement			3 years
Transaction fee valuation rate (as a percent)			1.00%
Board of Directors Chairman | Services Agreement with MVB Management, LLC and THL Managers VIII, LLC
Related Party Transaction [Line Items]
Fees/ expenses with related party included in "Selling and Administrative Expenses"									0.0	$ 29.1
Board of Directors Chairman | Stock Issuance Costs | Bilcar
Related Party Transaction [Line Items]
Expenses from transactions with related party	2.5
Board of Directors Chairman | Equity Commitment Fee | Cannae Holdings
Related Party Transaction [Line Items]
Expenses from transactions with related party				$ 12.0
Board of Directors Chairman | Service Agreement, Transaction Fees | Trasimene Capital Management, LLC
Related Party Transaction [Line Items]
Expenses from transactions with related party									0.4
Director
Related Party Transaction [Line Items]
Stock options granted during period (shares) | shares		2,080,000
Director | Stock Issuance Costs | CC Star Holdings, LP
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 2.5
Management | Travel Costs
Related Party Transaction [Line Items]
Fees/ expenses with related party included in "Selling and Administrative Expenses"							$ 0.1		0.5
Affiliated Entity | Private Placement | Common Stock
Related Party Transaction [Line Items]
Stock issued (shares) | shares	18,458,700
Issuance price per share relative to IPO price per share (as a percent)	98.50%
Exercise price of stock options granted during period (USD per share) | $ / shares		$ 22.00
Affiliated Entity | Cannae Holdings | Private Placement | Common Stock
Related Party Transaction [Line Items]
Gross proceeds from sale of stock	$ 200.0
Affiliated Entity | Black Knight Inc. | Private Placement | Common Stock
Related Party Transaction [Line Items]
Gross proceeds from sale of stock	100.0
Affiliated Entity | CC Capital | Private Placement | Common Stock
Related Party Transaction [Line Items]
Gross proceeds from sale of stock	100.0
Affiliated Entity | Stock Issuance Costs | THL Managers
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 2.5
Affiliated Entity | Equity Commitment Fee | THL Managers
Related Party Transaction [Line Items]
Expenses from transactions with related party				7.5
Chief Operating Officer | Motive Partners | London
Related Party Transaction [Line Items]
Term of lease												5 years
Chief Operating Officer | Motive Partners | New York
Related Party Transaction [Line Items]
Term of lease											1 year
Chief Operating Officer | Due Diligence Consulting Services Fee | Motive Partners
Related Party Transaction [Line Items]
Expenses from transactions with related party				$ 0.6
Chief Operating Officer | Lease Termination Fee | Motive Partners | London
Related Party Transaction [Line Items]
Fees/ expenses with related party included in "Selling and Administrative Expenses"					$ 0.1
Chief Operating Officer | Lease Cost | Motive Partners | London
Related Party Transaction [Line Items]
Expenses from transactions with related party									$ 1.0
Chief Operating Officer | Lease Cost | Motive Partners | New York
Related Party Transaction [Line Items]
Expenses from transactions with related party											$ 0.2