Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (25,474)	$ (19,704)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,279	3,904
Depreciation and amortization expense	1,730	386
Non-cash interest expense	3	54
Accretion of discount on investments	(59)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(62)	(1,040)
Deferred revenue	(3,015)	117,290
Accounts payable	(1,916)	1,331
Accrued expenses and other current liabilities	(1,135)	(806)
Net cash provided by (used in) operating activities	(25,649)	101,415
Cash flows from investing activities:
Purchases of property and equipment	(2,247)	(3,400)
Purchases of investments	(22,513)	(45,301)
Maturities of investments	37,751	66,425
Changes in restricted cash	(1)	(1)
Net cash provided by investing activities	12,990	17,723
Cash flows from financing activities:
Proceeds from exercise of stock options and common stock warrants	19	157
Net cash provided by financing activities	19	157
Net increase (decrease) in cash and cash equivalents	(12,640)	119,295
Cash and cash equivalents at beginning of period	54,539	73,933
Cash and cash equivalents at end of period	41,899	193,228
Supplemental disclosure of cash flow information:
Cash paid for interest		10
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 349	$ 3,395