Description of Business and Significant Accounting Policies - Revenue Recognition (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Description Of Business And Significant Accounting Policies [Line Items]
Deferred commissions balance, beginning of period	$ 18,408,000	$ 18,738,000	$ 18,738,000
Deferral of commissions earned	3,099,000	3,711,000
Recognition of commission expense	(4,073,000)	(4,016,000)
Impairment of deferred commissions		(56,000)
Deferred commissions balance, end of period	17,434,000	$ 18,377,000	18,408,000
Other long-term assets
Description Of Business And Significant Accounting Policies [Line Items]
Unbilled receivables, noncurrent	$ 790,000		977,000
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 | Appliance revenue | Maximum
Description Of Business And Significant Accounting Policies [Line Items]
Concentration risk, percentage	5.00%
Unbilled
Description Of Business And Significant Accounting Policies [Line Items]
Accounts receivable	$ 1,872,000		$ 2,458,000