Revenue from Contracts with Customers - Costs of Obtaining Contracts (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Prepaid expenses and other
Costs of Obtaining Contracts with Customers
Current portion of costs of obtaining contracts with customers	$ 3.8
Other noncurrent assets
Costs of Obtaining Contracts with Customers
Non-current portion of costs of obtaining contracts with customers	$ 15.0
Residential Subscription | Minimum
Costs of Obtaining Contracts with Customers
Costs of contracts with customers, amortization period	3 years
Residential Subscription | Maximum
Costs of Obtaining Contracts with Customers
Costs of contracts with customers, amortization period	4 years
Business Subscription | Minimum
Costs of Obtaining Contracts with Customers
Costs of contracts with customers, amortization period	6 years
Business Subscription | Maximum
Costs of Obtaining Contracts with Customers
Costs of contracts with customers, amortization period	7 years