Revenue from Contracts with Customers - Contract Liabilities (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Unearned service revenue
Costs of Obtaining Contracts with Customers
Current portion of contract liabilities	$ 3.6
Other noncurrent liabilities
Costs of Obtaining Contracts with Customers
Non-current portion of contract liabilities	$ 0.5
Residential Subscription
Costs of Obtaining Contracts with Customers
Contract liability, term of contract	5 months
Business Subscription
Costs of Obtaining Contracts with Customers
Contract liability, term of contract	30 days