Segment Information (Narrative) (Details) customer in Millions	Dec. 31, 2018 customer
Cable Communications [Member]
Segment Reporting Information [Line Items]
Total customer relationships	30.3
Cable Communications [Member] | High-speed Internet Customers [Member]
Segment Reporting Information [Line Items]
Number of customers	27.2
Cable Communications [Member] | Video Customers [Member]
Segment Reporting Information [Line Items]
Number of customers	22.0
Cable Communications [Member] | Voice Customers [Member]
Segment Reporting Information [Line Items]
Number of customers	11.4
Cable Communications [Member] | Security And Automation Customers [Member]
Segment Reporting Information [Line Items]
Number of customers	1.3
Cable Communications [Member] | Residential [Member]
Segment Reporting Information [Line Items]
Total customer relationships	28.0
Cable Communications [Member] | Business Customers [Member]
Segment Reporting Information [Line Items]
Total customer relationships	2.3
Sky Limited [Member]
Segment Reporting Information [Line Items]
Retail customer relationships	23.6