Revenue from Contracts with Customers - Revenue by Service Offering (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue from Contracts with Customers
Total revenue	$ 286.3	$ 284.5
Subscription services
Revenue from Contracts with Customers
Revenue	267.8	264.6
HSD
Revenue from Contracts with Customers
Revenue	152.7	136.6
Video
Revenue from Contracts with Customers
Revenue	93.3	103.4
Telephony
Revenue from Contracts with Customers
Revenue	21.8	24.6
Other business services
Revenue from Contracts with Customers
Revenue	6.2	6.5
Other business services - Wholesale and colocation lease revenue
Revenue from Contracts with Customers
Revenue	5.4	5.4
Other services
Revenue from Contracts with Customers
Other revenue	12.3	13.4
Residential Subscription
Revenue from Contracts with Customers
Total revenue	231.0	228.8
Residential Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	231.0	228.8
Residential Subscription | HSD
Revenue from Contracts with Customers
Revenue	129.8	115.3
Residential Subscription | Video
Revenue from Contracts with Customers
Revenue	89.6	99.5
Residential Subscription | Telephony
Revenue from Contracts with Customers
Revenue	11.6	14.0
Business Subscription
Revenue from Contracts with Customers
Total revenue	36.8	35.8
Business Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	36.8	35.8
Business Subscription | HSD
Revenue from Contracts with Customers
Revenue	22.9	21.3
Business Subscription | Video
Revenue from Contracts with Customers
Revenue	3.7	3.9
Business Subscription | Telephony
Revenue from Contracts with Customers
Revenue	$ 10.2	$ 10.6