Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) source	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]
Number of primary revenue sources | source	3
Service provider contract term (years)	1 year
Service provider renewal term (years)	1 year
Period for returns (up to one year)	1 year
Percentage of revenue reserved for returns (percent)	2.00%	2.00%	2.00%
Minimum
Deferred Revenue Arrangement [Line Items]
Subscriber contract term (years)	3 years
Maximum
Deferred Revenue Arrangement [Line Items]
Subscriber contract term (years)	5 years
Activation Fees
Deferred Revenue Arrangement [Line Items]
Deferred revenue | $	$ 10.5	$ 11.2
Activation Fees | Minimum
Deferred Revenue Arrangement [Line Items]
Deferred revenue recognition period	12 months
Activation Fees | Maximum
Deferred Revenue Arrangement [Line Items]
Deferred revenue recognition period	10 years