Investments In Unconsolidated Affiliates - Summarized Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Variable Interest Entity [Line Items]
Total assets						$ 291.3
Revenues	$ 312.6	$ 299.1		$ 896.4	$ 877.1
Loss before provision for income taxes and equity in net income of affiliates	43.9	58.6		178.9	168.4
Net earnings	114.6	37.3		203.8	95.9
Net earnings attributable to Black Knight	127.8	37.3		217.0	$ 95.9
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Current assets	723.4			723.4		417.9
Non-current assets	8,462.0			8,462.0		8,694.9
Total assets	9,185.4			9,185.4		9,112.8
Current liabilities, including short-term debt	785.7			785.7		1,090.4
Non-current liabilities	4,736.0			4,736.0		5,412.9
Cumulative preferred series A stock	5,521.7			5,521.7		6,503.3
Cumulative preferred series A stock	0.0			0.0		1,030.6
Total equity	3,663.7			3,663.7		1,578.9
Total liabilities and partners' capital	9,185.4			9,185.4		$ 9,112.8
Revenues	442.1	408.2	$ 981.2	1,258.0
Loss before provision for income taxes and equity in net income of affiliates	(24.9)	(79.8)	(278.1)	(227.8)
Net earnings	(14.9)	(55.3)	(197.2)	(114.8)
Net earnings attributable to Black Knight	$ (17.0)	$ (88.8)	$ (282.5)	$ (182.6)