Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (60,427)	$ (66,303)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	13,150	12,865
Depreciation and amortization expense	5,356	2,511
Non-cash interest expense	3	2
Accretion of discount on investments	(166)	(317)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(220)	(2,527)
Deferred revenue	(9,044)	111,545
Accounts payable	(2,502)	84
Accrued expenses and other current liabilities	(1,065)	6,866
Net cash provided by (used in) operating activities	(54,915)	64,726
Cash flows from investing activities:
Purchases of property and equipment	(3,991)	(15,805)
Purchases of investments	(75,728)	(245,728)
Sales and maturities of investments	126,125	176,230
Changes in restricted cash	(113)	117
Net cash provided by (used in) investing activities	46,293	(85,186)
Cash flows from financing activities:
Proceeds from exercise of stock options and common stock warrants	108	2,138
Net cash provided by financing activities	108	2,138
Net decrease in cash and cash equivalents	(8,514)	(18,322)
Cash and cash equivalents at beginning of period	54,539	73,933
Cash and cash equivalents at end of period	46,025	55,611
Supplemental disclosure of cash flow information:
Cash paid for interest	1	109
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 453	$ 4,484