The amount of the company's cash, cash equivalents, short-term investments and restricted cash.

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Line of Credit Facility, Minimum Required Unused Borrowing Capacity

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The amount of the company's working capital used to fund operations.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Fair value of the amount outstanding under the credit facility.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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The net result for the period of deducting operating expenses from operating revenues.

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No definition available.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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Amount of lessee's right to use underlying asset under operating lease.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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Basis of presentation and significant accounting policies (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Sep. 30, 2017
Concentration Risk [Line Items]
Working capital	$ 113,900,000		$ 113,900,000
Cash, cash equivalents, short-term investments and restricted cash	79,800,000		79,800,000
Operating losses	(1,261,000)	$ 8,689,000	18,892,000	$ 35,083,000
Net cash from operating activities			18,389,000	$ 8,997,000
Accumulated deficit	416,617,000		416,617,000			$ 397,472,000
Current portion of long-term debt	3,054,000		3,054,000			$ 2,897,000
Operating lease right-of-use assets	16,037,000		16,037,000		$ 17,300,000
Operating lease liability	$ 19,121,000		$ 19,121,000		$ 20,800,000
Huawei Technologies \| Revenue
Concentration Risk [Line Items]
Percentage of concentration of credit risk	37.00%	47.00%	41.00%	47.00%
Top Five Customers \| Revenue
Concentration Risk [Line Items]
Percentage of concentration of credit risk	85.00%	89.00%	84.00%	89.00%
Wells Fargo Bank, National Association \| Revolving Credit Facility
Concentration Risk [Line Items]
Credit facility, maximum borrowing amount							$ 50,000,000.0
Wells Fargo Credit Facility \| Line of Credit
Concentration Risk [Line Items]
Credit facility, maximum borrowing amount							$ 50,000,000.0
Line of credit fair value outstanding	$ 32,000,000.0		$ 32,000,000.0
Unused part of credit facility	12,400,000		12,400,000
Minimum required unused borrowing capacity	$ 5,000,000.0		$ 5,000,000.0