Commitments and Contingencies (Narrative) (Details) € in Millions, £ in Millions	1 Months Ended				3 Months Ended				12 Months Ended
	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2015 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 USD ($)	Nov. 26, 2007 association
Loss Contingencies [Line Items]
Programming costs					$ 437,400,000	$ 402,700,000
Interkabel Acquisition
Loss Contingencies [Line Items]
Damages sought			€ 1,400.0	$ 1,600,000,000
Interkabel Acquisition
Loss Contingencies [Line Items]
Number of associations of municipalities in Belgium | association																	4
Loss contingency damages in excess value															€ 20.0	$ 22,400,000
Loss contingency accrual														$ 0
Deutsche Telekom Litigation
Loss Contingencies [Line Items]
Damages sought									€ 75.0	$ 84,000,000
Reduction of annual lease fees					83.33%
Belgium Regulatory Developments
Loss Contingencies [Line Items]
Percent of reduction in monthly wholesale cable resale access prices													17.00%	17.00%
Virgin Media VAT Matters
Loss Contingencies [Line Items]
Loss contingency accrual														$ 0
Virgin Media VAT Matters | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss													£ 47.0	$ 61,000,000
Virgin Media VAT Legislation Change
Loss Contingencies [Line Items]
Continuing operations							£ 67.0	$ 99,100,000
Provisions for litigation	£ 63.7	$ 83,100,000
Interest expense	£ 3.3	$ 4,400,000
Other Regulatory Issues | Maximum | Forecast | U.K.
Loss Contingencies [Line Items]
Estimate of possible loss											£ 28.0	$ 36,000,000