Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (49,205)	$ (36,658)	$ (31,778)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	3,887	2,507	1,717
Stock-based compensation expense	36,637	26,143	15,760
Accretion of discount on short-term investments	(1,257)	(116)	(231)
Non-cash loss on extinguishment of debt			1,156
Payment of original issue discount			(1,500)
Provision for (recovery of) doubtful accounts	(501)	293	909
Write-down of inventory	2,005	3,984	4,056
Loss on disposal of equipment			287
Non-cash interest expense	7,382	6,884	3,681
Unrealized gains (losses) on foreign currency transactions	1,188	(799)	1,854
Changes in operating assets and liabilities
Accounts receivable	(12,214)	(13,899)	(32,181)
Inventories	3,846	(16,297)	(27,031)
Prepaid expenses and other current assets	(767)	(501)	(1,997)
Other assets	(2,601)	(892)	(505)
Accounts payable	5,170	2,162	(5,586)
Accrued liabilities	(242)	12,269	12,136
Other long-term liabilities	964	647	750
Net cash used in operating activities	(5,708)	(14,273)	(58,503)
Cash flows from investing activities
Purchases of short-term investments	(213,723)	(287,938)	(372,309)
Proceeds from sales of short-term investments		5,993
Proceeds from maturity of short-term investments	228,373	290,341	243,890
Purchases of property and equipment	(8,217)	(4,327)	(3,368)
Net cash provided by (used in) investing activities	6,433	4,069	(131,787)
Cash flows from financing activities
Proceeds from issuance of convertible notes			172,500
Convertible notes initial issuance discount and debt issuance costs			(6,171)
Proceeds from issuance of warrants			33,120
Purchase of convertible note hedges			(45,092)
Repayment of debt			(19,500)
Minimum tax withholding paid on behalf of employees for net share settlement	(1,742)	(991)
Proceeds from issuance of common stock to employees	9,496	12,190	10,307
Net cash provided by financing activities	7,754	11,199	145,164
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(258)	444	(604)
Net increase (decrease) in cash, cash equivalents and restricted cash	8,221	1,439	(45,730)
Cash, cash equivalents and restricted cash
Cash, cash equivalents and restricted cash at beginning of year	43,651	42,212	87,942
Cash, cash equivalents and restricted cash at end of year	51,872	43,651	42,212
Supplemental disclosures of cash flow information
Cash paid for income taxes	1,847	687	492
Cash paid for interest	3,019	3,019	2,469
Significant non-cash transactions
Purchases of property and equipment in accounts payable	$ 245	592	725
Vesting of early-exercised stock options		$ 7	$ 47