Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (53,492)	$ (47,617)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,781	8,621
Depreciation and amortization expense	3,490	1,260
Non-cash interest expense	2	1
Accretion of discount on investments	(116)	(222)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	287	(2,719)
Deferred revenue	(6,029)	114,383
Accounts payable	(3,651)	395
Accrued expenses and other current liabilities	(577)	1,991
Net cash provided by (used in) operating activities	(51,305)	76,093
Cash flows from investing activities:
Purchases of property and equipment	(3,527)	(9,946)
Purchases of investments	(43,795)	(206,534)
Sales and maturities of investments	77,807	125,335
Changes in restricted cash	(113)	(1)
Net cash provided by (used in) investing activities	30,372	(91,146)
Cash flows from financing activities:
Proceeds from exercise of stock options	36	944
Net cash provided by financing activities	36	944
Net decrease in cash and cash equivalents	(20,897)	(14,109)
Cash and cash equivalents at beginning of period	54,539	73,933
Cash and cash equivalents at end of period	33,642	59,824
Supplemental disclosure of cash flow information:
Cash paid for interest		108
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 140	$ 4,899