Revenue from Contracts with Customers - Revenue by Service Offering (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from Contracts with Customers
Total revenue	$ 288.7	$ 285.4	$ 855.2	$ 862.3
Subscription services
Revenue from Contracts with Customers
Revenue	269.5	262.4	798.0	794.0
HSD
Revenue from Contracts with Customers
Revenue	142.6	130.2	416.5	388.1
Video
Revenue from Contracts with Customers
Revenue	103.9	106.4	309.8	327.0
Telephony
Revenue from Contracts with Customers
Revenue	23.0	25.8	71.7	78.9
Other business services
Revenue from Contracts with Customers
Revenue	6.3	6.8	19.2	21.0
Other business services - Wholesale and colocation lease revenue
Revenue from Contracts with Customers
Revenue	5.4	5.6	16.1	16.5
Other services
Revenue from Contracts with Customers
Other revenue	12.9	16.2	38.0	47.3
Residential Subscription
Revenue from Contracts with Customers
Total revenue	233.6	227.7	690.8	691.3
Residential Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	233.6	227.7	690.8	691.3
Residential Subscription | HSD
Revenue from Contracts with Customers
Revenue	120.8	109.8	352.0	328.4
Residential Subscription | Video
Revenue from Contracts with Customers
Revenue	100.1	102.8	298.6	316.1
Residential Subscription | Telephony
Revenue from Contracts with Customers
Revenue	12.7	15.1	40.2	46.8
Business Subscription
Revenue from Contracts with Customers
Total revenue	35.9	34.7	107.2	102.7
Business Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	35.9	34.7	107.2	102.7
Business Subscription | HSD
Revenue from Contracts with Customers
Revenue	21.8	20.4	64.5	59.7
Business Subscription | Video
Revenue from Contracts with Customers
Revenue	3.8	3.6	11.2	10.9
Business Subscription | Telephony
Revenue from Contracts with Customers
Revenue	$ 10.3	$ 10.7	$ 31.5	$ 32.1