Revenue from Contracts with Customers - Revenue by Service Offering (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue from Contracts with Customers
Total revenue	$ 282.0	$ 289.7	$ 566.5	$ 576.9
Subscription services
Revenue from Contracts with Customers
Revenue	263.9	266.4	528.5	531.6
HSD
Revenue from Contracts with Customers
Revenue	137.3	130.4	273.9	257.9
Video
Revenue from Contracts with Customers
Revenue	102.5	109.6	205.9	220.6
Telephony
Revenue from Contracts with Customers
Revenue	24.1	26.4	48.7	53.1
Other business services
Revenue from Contracts with Customers
Revenue	6.4	7.1	12.9	14.2
Other business services - Wholesale and colocation lease revenue
Revenue from Contracts with Customers
Revenue	5.3	5.4	10.7	10.9
Other services
Revenue from Contracts with Customers
Other revenue	11.7	16.2	25.1	31.1
Residential Subscription
Revenue from Contracts with Customers
Total revenue	228.4	232.1	457.2	463.6
Residential Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	228.4	232.1	457.2	463.6
Residential Subscription | HSD
Revenue from Contracts with Customers
Revenue	115.9	110.5	231.2	218.6
Residential Subscription | Video
Revenue from Contracts with Customers
Revenue	99.0	105.9	198.5	213.3
Residential Subscription | Telephony
Revenue from Contracts with Customers
Revenue	13.5	15.7	27.5	31.7
Business Subscription
Revenue from Contracts with Customers
Total revenue	35.5	34.3	71.3	68.0
Business Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	35.5	34.3	71.3	68.0
Business Subscription | HSD
Revenue from Contracts with Customers
Revenue	21.4	19.9	42.7	39.3
Business Subscription | Video
Revenue from Contracts with Customers
Revenue	3.5	3.7	7.4	7.3
Business Subscription | Telephony
Revenue from Contracts with Customers
Revenue	$ 10.6	$ 10.7	$ 21.2	$ 21.4