Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (161,466)	$ (113,192)	$ (89,631)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization expense	18,517	15,949	10,600
Stock-based compensation	22,202	19,524	11,176
Provision for doubtful accounts on accounts receivable			171
Amortization of premiums and discounts on marketable securities	17	248	33
Loss on extinguishment of debt	283
Gain on disposal of long-lived assets	(327)
Changes in operating assets and liabilities:
Accounts receivable	(11,721)	(844)	2,361
Receivable from Roche	(8,152)	(1,604)	(403)
Inventory	(2,576)	(2,446)	(3,183)
Prepaid expenses and other current assets	(3,825)	1,305	(3,652)
Other assets	210	210	(267)
Accounts payable	8,434	5,060	(1,559)
Accrued expenses and other current liabilities	15,463	8,919	4,704
Deferred rent and other non-current liabilities	1,848	(1,688)	1,805
Deferred revenue	107	1,387	377
Roche related-party deferred revenue	(5)	5	3,742
Net cash used in operating activities	(120,991)	(67,167)	(63,726)
Investing activities
Purchases of property and equipment	(15,658)	(20,879)	(25,430)
Purchases of marketable securities and other investments	(4,996)	(107,456)	(114,756)
Proceeds from maturities of marketable securities	84,390	141,023
Change in restricted cash	(910)		(531)
Net cash provided by (used in) investing activities	62,826	12,688	(140,717)
Financing activities
Proceeds from indebtedness to Roche	60,000
Proceeds from issuance of restricted stock and stock option exercises	5,963	652	4,739
Payment of debt issuance costs	(14)	(319)
Net cash provided by financing activities	65,949	333	250,126
Effect of exchange rate changes on cash and cash equivalents	3
Net increase (decrease) in cash and cash equivalents	7,787	(54,146)	45,683
Cash and cash equivalents at beginning of period	63,617	117,763	72,080
Cash and cash equivalents at end of period	71,404	63,617	117,763
Supplemental disclosure of cash flow information
Cash paid for interest	545	77
Supplemental disclosure of non-cash investing and financing activities
Acquisition of property and equipment included in accounts payable and accrued expenses	$ 4,098	$ 2,029	6,822
Roche [Member]
Financing activities
Proceeds from issuance of common stock, net of issuance costs			$ 245,387