Investments (Schedule of Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Equity	$ 3,367.7	$ 3,488.7
Short-term, separately-managed accounts (SMAs)	1,571.7	1,600.2
Long-term, SMAs	389.5	365.7
Total investments	3,629.0	3,466.0
Total investments (g)	6,996.7	6,954.7
Short-term investments	1,571.7	1,600.2
Long-term investments	5,425.0	5,354.5
VodafoneZiggo JV
Schedule of Investments [Line Items]
Equity	$ 2,924.8	3,052.3
Ownership percentage	50.00%
All3Media Group (All3Media)
Schedule of Investments [Line Items]
Equity	$ 160.3	157.7
Ownership percentage	50.00%
Formula E Holdings Ltd (Formula E)
Schedule of Investments [Line Items]
Equity	$ 110.2	105.8
Ownership percentage	32.90%
Other
Schedule of Investments [Line Items]
Equity	$ 172.4	172.9
Long-term investments at fair value	333.3	299.4
ITV
Schedule of Investments [Line Items]
Long-term investments at fair value	$ 660.8	581.0
Ownership percentage	9.90%
Skillz Inc.
Schedule of Investments [Line Items]
Long-term, SMAs	$ 198.3	225.4
Ownership percentage	2.80%
Plume Design, Inc. (Plume)
Schedule of Investments [Line Items]
Long-term, SMAs	$ 110.0	54.9
Ownership percentage	12.10%
Univision Holdings Inc.
Schedule of Investments [Line Items]
Long-term, SMAs	$ 100.0	100.0
Ownership percentage	11.50%
Lions Gate Entertainment Corp (Lionsgate)
Schedule of Investments [Line Items]
Long-term investments at fair value	$ 92.8	72.0
Ownership percentage	3.00%
CANAL+ Polska S.A.
Schedule of Investments [Line Items]
Long-term investments at fair value	$ 87.5	92.3
Ownership percentage	17.00%
EdgeConneX Inc. (EdgeConneX)
Schedule of Investments [Line Items]
Long-term investments at fair value	$ 85.1	$ 75.1
Ownership percentage	5.10%

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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