Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (99,092)	$ (51,232)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,187	1,189
Noncash interest expense	27	59
Stock-based compensation	8,818	4,494
Accretion of premiums and discounts on investments	(34)	204
Changes in assets and liabilities:
Unbilled accounts receivable	142	(41)
Prepaid expenses and other current assets	(172)	710
Other assets	(800)	(5)
Accounts payable	(125)	(373)
Accrued expenses	9,076	1,927
Deferred revenue	(10,463)	37,707
Deferred rent	6,321	(355)
Net cash used in operating activities	(85,115)	(5,716)
Investing activities
Purchases of property and equipment	(4,932)	(2,336)
Restricted cash	(3,500)	119
Purchases of investments	(315,288)	(105,318)
Maturities of investments	198,000	27,000
Net cash (used in) investing activities	(125,720)	(80,535)
Financing activities
Principal payments on loan payable	(2,000)	(2,500)
Payment of offering costs	(923)
Proceeds from public offering of common stock, net of commissions and underwriting discounts	216,200
Proceeds from issuance of common stock, net of repurchases	2,818	447
Net cash provided by (used in) financing activities	216,095	(2,053)
Net increase (decrease) in cash and cash equivalents	5,260	(88,304)
Cash and cash equivalents at beginning of period	52,069	162,707
Cash and cash equivalents at end of period	57,329	74,403
Supplemental cash flow information
Public offering costs incurred but unpaid at period end	100
Property and equipment purchases unpaid at period end	3,196	15
Cash paid for interest	$ 125	$ 254