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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Present value of lessee's discounted obligation for lease payments from 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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Sep. 30, 2017
Concentration Risk [Line Items]
Working capital	$ 108,100,000		$ 108,100,000
Cash, cash equivalents, short-term investments and restricted cash	74,200,000		74,200,000
Operating losses	8,224,000	$ 9,750,000	20,153,000	$ 26,394,000
Net cash from operating activities			9,433,000	$ (4,534,000)
Accumulated deficit	418,889,000		418,889,000			$ 397,472,000
Current portion of long-term debt	3,048,000		3,048,000			$ 2,897,000
Operating lease right-of-use assets	16,520,000		16,520,000		$ 17,300,000
Operating lease liability	$ 19,800,000		$ 19,800,000		$ 20,800,000
Huawei Technologies \| Revenue
Concentration Risk [Line Items]
Percentage of concentration of credit risk	36.00%	43.00%	43.00%	46.00%
Top Five Customers \| Revenue
Concentration Risk [Line Items]
Percentage of concentration of credit risk	84.00%	90.00%	85.00%	87.00%
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
Line of credit fair value outstanding	$ 31,700,000		$ 31,700,000
Unused part of credit facility	13,700,000		13,700,000
Minimum required unused borrowing capacity	$ 5,000,000.0		$ 5,000,000.0