Summary of Significant Accounting Policies - Revenue Recognition (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) source		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)
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
Total revenue	$ 111,402,000	$ 111,848,000	$ 104,488,000	$ 92,756,000	$ 88,793,000	$ 89,962,000	$ 85,988,000	$ 74,194,000	$ 420,494,000		$ 338,937,000		$ 261,106,000
Period for returns (up to one year)									1 year
Percentage of revenue reserved for returns (percent)									2.00%		2.00%		2.00%
Deferred revenue for contract liabilities	11,176,000				12,678,000				$ 11,176,000		$ 12,678,000		$ 12,625,000
Amortization of capitalized commission costs									1,983,000
Total cost of revenue	38,858,000	$ 41,992,000	$ 36,488,000	$ 28,377,000	27,885,000	$ 31,833,000	$ 29,835,000	$ 26,635,000	$ 145,715,000	[1]	116,188,000	[1]	99,380,000	[1]
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 for contract liabilities	$ 9,200,000				$ 10,500,000				$ 9,200,000		10,500,000
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
Software License Revenue
Deferred Revenue Arrangement [Line Items]
Total cost of revenue									$ 1,700,000		1,200,000		0
Commission
Deferred Revenue Arrangement [Line Items]
Amortization of capitalized commission costs									$ 2,000,000		$ 0		$ 0

[1] Exclusive of amortization and depreciation shown in operating expenses below.