Segment Reporting (Geographic Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenue	$ 2,958.1	$ 2,929.0	$ 9,097.7	$ 8,373.7
VodafoneZiggo JV
Segment Reporting Information [Line Items]
Revenue	1,138.1	1,167.2	3,468.5	3,335.1
Operating Segments | U.K.
Segment Reporting Information [Line Items]
Revenue	1,542.9	1,495.7	4,792.9	4,350.9
Operating Segments | Belgium
Segment Reporting Information [Line Items]
Revenue	746.8	759.1	2,260.3	2,106.5
Operating Segments | Switzerland
Segment Reporting Information [Line Items]
Revenue	323.3	352.8	1,000.4	1,023.0
Operating Segments | Ireland
Segment Reporting Information [Line Items]
Revenue	124.8	121.4	387.9	336.7
Operating Segments | Poland
Segment Reporting Information [Line Items]
Revenue	107.6	107.5	334.0	305.2
Operating Segments | Slovakia
Segment Reporting Information [Line Items]
Revenue	15.5	15.6	47.8	44.5
Other, including intersegment eliminations (a)
Segment Reporting Information [Line Items]
Revenue	$ 97.2	$ 76.9	$ 274.4	$ 206.9