Revenue from Contracts with Customers - Revenue by Service Offering (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from Contracts with Customers
Total revenue	$ 291.6	$ 297.8	$ 868.4	$ 895.3
Subscription services
Revenue from Contracts with Customers
Revenue	267.3		800.6
HSD
Revenue from Contracts with Customers
Revenue	119.0		348.2
Video
Revenue from Contracts with Customers
Revenue	119.3		364.2
Telephony
Revenue from Contracts with Customers
Revenue	29.0		88.2
Other business services
Revenue from Contracts with Customers
Revenue	6.6		20.6
Other business services - Wholesale and collocation revenue
Revenue from Contracts with Customers
Revenue	5.0		16.0
Other services
Revenue from Contracts with Customers
Other revenue	17.7		47.2
Residential Subscription
Revenue from Contracts with Customers
Total revenue	234.3		703.6
Residential Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	234.3		703.6
Residential Subscription | HSD
Revenue from Contracts with Customers
Revenue	100.2		293.5
Residential Subscription | Video
Revenue from Contracts with Customers
Revenue	115.8		353.6
Residential Subscription | Telephony
Revenue from Contracts with Customers
Revenue	18.3		56.5
Business Subscription
Revenue from Contracts with Customers
Total revenue	33.0		97.0
Business Subscription | Subscription services
Revenue from Contracts with Customers
Revenue	33.0		97.0
Business Subscription | HSD
Revenue from Contracts with Customers
Revenue	18.8		54.7
Business Subscription | Video
Revenue from Contracts with Customers
Revenue	3.5		10.6
Business Subscription | Telephony
Revenue from Contracts with Customers
Revenue	$ 10.7		$ 31.7