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Investments (Schedules) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Investments (Schedules) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment [Line Items]
Equity	$ 4,045.9		$ 4,045.9		$ 4,324.6
Fair value	1,476.4		1,476.4		2,315.3
Cost	0.0		0.0
Cost					31.5
Total	5,522.3		5,522.3		6,671.4
Share of results of affiliates, net	(11.1)	$ (26.8)	(129.9)	$ (46.1)
VodafoneZiggo JV
Investment [Line Items]
Equity	3,876.2		3,876.2		4,162.8
Share of results of affiliates, net	(8.5)	(23.4)	(98.5)	(18.2)
Summarized results of operations of the VodafoneZiggo JV
Revenue	1,138.1	1,167.2	3,468.5	3,335.1
Loss before income taxes	(96.1)	(115.7)	(381.6)	(181.2)
Net loss	(74.9)	(79.6)	(283.1)	(125.9)
Other
Investment [Line Items]
Equity	169.7		169.7		161.8
Fair value	307.0		307.0		244.7
Share of results of affiliates, net	(2.6)	$ (3.4)	(31.4)	$ (27.9)
ITV plc (ITV) — subject to re-use rights
Investment [Line Items]
Fair value	820.4		820.4		892.0
Sumitomo Corporation (Sumitomo)
Investment [Line Items]
Fair value	0.0		0.0		776.5
ITI Neovision S.A. (ITI Neovision)
Investment [Line Items]
Fair value	164.4		164.4		161.9
Lions Gate Entertainment Corp (Lionsgate)
Investment [Line Items]
Fair value	119.2		119.2		163.9
Casa Systems, Inc. (Casa)
Investment [Line Items]
Fair value	$ 65.4		$ 65.4		$ 76.3