Investments (Schedule of Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Equity	$ 3,624.7	$ 3,492.8
Short-term, separately-managed accounts (SMAs)	2,549.3	0.0
Long-term, SMAs	439.2	0.0
Total investments	3,929.9	1,289.2
Total investments	7,554.6	4,782.0
Short-term investments	2,549.3	0.0
Long-term investments	5,005.3	4,782.0
VodafoneZiggo JV
Schedule of Investments [Line Items]
Equity	3,241.5	3,174.1
All3Media
Schedule of Investments [Line Items]
Equity	125.0	172.8
Formula E Holdings Ltd (Formula E)
Schedule of Investments [Line Items]
Equity	93.4	105.2
Other
Schedule of Investments [Line Items]
Equity	164.8	40.7
Long-term investments at fair value	415.1	300.7
ITV
Schedule of Investments [Line Items]
Long-term investments at fair value	347.9	798.1
ITI Neovision S.A. (ITI Neovision)
Schedule of Investments [Line Items]
Long-term investments at fair value	118.2	122.4
Lionsgate
Schedule of Investments [Line Items]
Long-term investments at fair value	60.2	$ 68.0
Third-Party Purchaser
Schedule of Investments [Line Items]
Noncontrolling interest	$ 9.2