Statement of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flow from operating activities
Net loss	$ (58,869)	$ (25,520)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,945	4,528
Depreciation	1,159	448
Amortization of premiums and discounts on marketable securities	197	571
In-kind research and development expenses	113	989
Other non-cash items	139	(409)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,764	(311)
Other non-current assets		57
Deferred revenue	(3,790)	(12,219)
Accounts payable	361	175
Accrued expenses	3,950	2,301
Other non-current liabilities	1,000
Lease incentive benefit	515
Net cash used in operating activities	(46,516)	(29,390)
Cash flow from investing activities
Purchases of property and equipment	(3,824)	(1,160)
Change in restricted cash		(471)
Purchases of marketable securities and warrants	(49,856)	(66,627)
Proceeds from maturities of marketable securities	110,100	129,700
Net cash provided by investing activities	56,420	61,442
Cash flow from financing activities
Proceeds from the exercise of stock options and issuance of restricted stock	1,656	146
Net cash provided by financing activities	1,656	146
Net increase in cash and cash equivalents	11,560	32,198
Cash and cash equivalents, beginning of period	36,641	31,309
Cash and cash equivalents, end of period	$ 48,201	$ 63,507