Revenue from Contracts with Customers - Revenue by Service Offering (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from Contracts with Customers
Other revenue	$ 53.0	$ 62.7	$ 62.9
Total revenue	1,148.4	1,145.8	1,153.8
Subscription services
Revenue from Contracts with Customers
Revenue	1,070.1	1,055.7	1,062.8
Other business services
Revenue from Contracts with Customers
Revenue	25.3	27.4	28.1
Other business services - Wholesale and colocation lease revenue
Revenue from Contracts with Customers
Revenue	21.3	21.6	21.9
Residential Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	926.4	918.0	933.3
Residential Subscription | HSD
Revenue from Contracts with Customers
Revenue	480.1	440.7	393.6
Residential Subscription | Video
Revenue from Contracts with Customers
Revenue	393.8	417.4	465.3
Residential Subscription | Telephony
Revenue from Contracts with Customers
Revenue	52.5	59.9	74.4
Business Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	143.7	137.7	129.5
Business Subscription | HSD
Revenue from Contracts with Customers
Revenue	87.1	80.3	73.5
Business Subscription | Video
Revenue from Contracts with Customers
Revenue	15.0	14.6	14.1
Business Subscription | Telephony
Revenue from Contracts with Customers
Revenue	$ 41.6	$ 42.8	$ 41.9