Balance Sheet Components - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 15,889	$ 12,736	$ 10,748
Less: accumulated depreciation and amortization	(10,593)	(9,805)	(8,962)
Property and equipment, net	5,296	2,931	1,786
Prototype
Property Plant And Equipment [Line Items]
Property and equipment, gross	6,471	6,342	6,053
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,605	4,214	2,714
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,274	1,270	1,154
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	347	263	232
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	790	$ 647	$ 595
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,402

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.13)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 13
-Subparagraph a
-Article 5

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-Name Regulation S-X (SX)
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