Basis of Presentation (Narrative) (Details)	9 Months Ended
	Sep. 30, 2018 market country	Dec. 31, 2017	Dec. 31, 2016
Consumer and Business-to-Business Communication Services | Latin America and the Caribbean
Basis of Presentation [Line Items]
Number of countries in which entity provides services | country	18
Sub-Sea Networks | Cable & Wireless Communications Limited (C&W)
Basis of Presentation [Line Items]
Number of markets | market	40
Telenet
Basis of Presentation [Line Items]
Percentage ownership in subsidiary	58.70%
VodafoneZiggo JV
Basis of Presentation [Line Items]
Ownership percentage	50.00%	50.00%	50.00%
Liberty Puerto Rico
Basis of Presentation [Line Items]
Ownership percentage	60.00%