Description of Business and Significant Accounting Policies - Revenue Recognition (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Description Of Business And Significant Accounting Policies [Line Items]
Accounts receivable, net	$ 44,564,000		$ 44,564,000		$ 60,994,000
Unbilled receivables, current	2,200,000		2,200,000		1,400,000
Deferred commissions balance, beginning of period	17,727,000	$ 16,928,000	17,331,000	$ 18,408,000	18,408,000
Deferral of commissions earned	4,271,000	4,158,000	12,862,000	10,953,000
Recognition of commission expense	(4,867,000)	(4,095,000)	(12,844,000)	(12,370,000)
Impairment of deferred commissions	(21,000)		(239,000)
Deferred commissions balance, end of period	17,110,000	$ 16,991,000	17,110,000	$ 16,991,000	17,331,000
Other long-term assets
Description Of Business And Significant Accounting Policies [Line Items]
Unbilled receivables, noncurrent	$ 858,000		$ 858,000		$ 592,000
Minimum
Description Of Business And Significant Accounting Policies [Line Items]
Payment terms			30 days
Maximum
Description Of Business And Significant Accounting Policies [Line Items]
Payment terms			60 days
Total revenue | Product | Appliance | Maximum
Description Of Business And Significant Accounting Policies [Line Items]
Concentration risk, percentage			5.00%