Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (41,358)	$ (91,353)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,592	1,116
Stock-based compensation expense	6,839	9,852
Goodwill impairment charge		49,514
Amortization of premium and accrued interest on marketable securities	419
Non-cash research and development expense	60	24
Changes in operating assets and liabilities:
Receivable from collaborative partner	886	(532)
Prepaid expenses and other current assets	(136)	(1,317)
Accounts payable	166	817
Accrued expenses and other current liabilities	(208)	(46)
Restructuring liabilities	(25)	(988)
Deferred revenue	(1,388)	2,936
Deferred rent	(70)	(47)
Net cash used in operating activities	(33,223)	(30,024)
Cash flows from investing activities:
Purchases of investments	(201,038)
Maturities of investments	45,061	37,738
Purchases of property and equipment	(918)	(1,488)
Cash acquired in business acquisition		3,449
Net cash provided by (used in) investing activities	(156,895)	39,699
Cash flows from financing activities:
Proceeds from issuance of common stock pursuant to option exercises	312	633
Taxes paid related to net share settlement of restricted stock units	(292)	(493)
Proceeds from employee stock purchase plan	83	113
Proceeds from financing arrangement		100
Net cash provided by financing activities	103	353
Effect of foreign currency exchange rate on cash and cash equivalents	(442)	(105)
Net increase (decrease) in cash and cash equivalents	(190,457)	9,923
Cash and cash equivalents at beginning of period	222,170	221,348
Cash and cash equivalents at end of period	31,713	231,271
Supplemental schedule of noncash investing and financing information
Unpaid deferred offering costs in accounts payable and accrued expenses	200
Issuance of common stock and exchange of stock options for business combination		64,845
Fixed assets in accounts payable, accrued expenses and other current liabilities	$ 148	$ 771