Investments (Schedule of Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Equity	$ 3,433.5	$ 3,492.8
Short-term, separately-managed accounts (SMAs)	1,640.2	0.0
Long-term, SMAs	333.8	0.0
Total investments	2,774.3	1,289.2
Total investments (f)	6,207.8	4,782.0
Short-term investments	1,640.2	0.0
Long-term investments	4,567.6	4,782.0
VodafoneZiggo JV
Schedule of Investments [Line Items]
Equity	3,131.7	3,174.1
All3Media
Schedule of Investments [Line Items]
Equity	141.8	172.8
Formula E Holdings Ltd (Formula E)
Schedule of Investments [Line Items]
Equity	101.8	105.2
Other
Schedule of Investments [Line Items]
Equity	58.2	40.7
Long-term investments at fair value	323.9	300.7
ITV plc (ITV) — subject to re-use rights
Schedule of Investments [Line Items]
Long-term investments at fair value	326.2	798.1
ITI Neovision S.A. (ITI Neovision)
Schedule of Investments [Line Items]
Long-term investments at fair value	111.6	122.4
Lions Gate Entertainment Corp (Lionsgate)
Schedule of Investments [Line Items]
Long-term investments at fair value	$ 38.6	$ 68.0