Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (167,211)	$ (157,803)	$ (100,048)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of fixed assets	11,309	7,600	3,710
Loss on disposal of property and equipment	151	40	49
Stock-based compensation	23,732	18,050	11,520
Amortization of deferred license fees			(294)
Amortization of other liabilities	(1,013)	(573)
Amortization of deferred financing costs	52	44
Forgiveness of long-term debt		(1,000)
Amortization of premium on short-term investments	463	1,323	842
Amortization of intangible assets	200	150
Proceeds from refundable tax credits	800
Changes in assets and liabilities:
Accounts receivable, net	(3,593)	(3,557)	(1,376)
Inventory, net	(156)	(2,660)	(4,017)
Prepaid expenses and other current assets	761	(3,057)	(1,329)
Accounts payable	(2,598)	661	1,886
Accrued liabilities	7,349	7,424	8,064
Lease incentive obligation	(312)	(553)	(487)
Accrued interest		(106)	22
Net cash used in operating activities	(130,066)	(134,017)	(81,458)
Cash flows from investing activities:
Purchases of marketable securities	(189,989)	(205,054)	(209,471)
Maturities of marketable securities	193,321	162,283	104,172
Purchases of property and equipment	(14,851)	(20,084)	(11,991)
Purchases of intangible assets		(1,900)
Net cash used in investing activities	(11,519)	(64,755)	(117,290)
Cash flows from financing activities:
Proceeds from exercise of common stock options	3,411	1,248	2,640
Proceeds from sale of common stock, net of issuance costs	144,242	174,140	238,580
Payments on capital lease obligations		(360)	(351)
Proceeds from mortgage payable		5,062
Payments on mortgage payable	(166)	(44)
Proceeds from New Market Tax Credit financing agreements			2,399
Proceeds in connection with the Company's employee stock purchase plan	2,099	1,719	760
Net cash provided by financing activities	149,586	181,765	244,028
Effects of exchange rate on cash and cash equivalents	(215)	11
Net increase (decrease) in cash and cash equivalents	7,786	(16,996)	45,280
Cash and cash equivalents, beginning of period	41,135	58,131
Cash and cash equivalents, end of period	48,921	41,135	58,131
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment acquired but not paid	655	1,705	546
Unrealized gain on available-for-sale investments	230	(329)	(240)
Issuance of 341,507, 21,826 and 32,666 shares of common stock to fund the Company's 401(k) matching contribution for 2015, 2014 and 2013, respectively	2,151	836	456
Interest paid	$ 209	$ 95	$ 29