Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (27,977)	$ (15,504)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	398	386
Noncash interest expense	11	22
Stock-based compensation	2,240	1,102
Accretion of premiums and discounts on investments	86	35
Changes in assets and liabilities:
Unbilled accounts receivable	751	(818)
Prepaid expenses and other current assets	(1,972)	2,262
Other assets	14	11
Accounts payable	71	567
Accrued expenses	(1,552)	(321)
Deferred revenue	(3,015)	43,127
Deferred rent	(135)	(118)
Net cash (used in) provided by operating activities	(31,080)	30,751
Investing activities
Purchases of property and equipment	(210)	(1,262)
Restricted cash		119
Purchases of investments	(18,003)	(69,222)
Maturities of investments	54,000
Net cash provided by (used in) investing activities	35,787	(70,365)
Financing activities
Principal payments on loan payable	(833)	(833)
Payment of offering costs	(489)
Proceeds from issuance of common stock, net of repurchases	902	29
Net cash used in financing activities	(420)	(804)
Net increase (decrease) in cash and cash equivalents	4,287	(40,418)
Cash and cash equivalents at beginning of period	52,069	162,707
Cash and cash equivalents at end of period	56,356	122,289
Supplemental cash flow information
Public offering costs incurred but unpaid at period end	516
Property and equipment purchases unpaid at period end	80	244
Cash paid for interest	$ 50	$ 95