Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flow from operating activities
Net loss	$ (65,757)	$ (58,869)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	13,072	6,945
Depreciation	1,538	1,159
Amortization of premiums and discounts on marketable securities	(1,563)	197
In-kind research and development expenses	176	113
Other non-cash items	497	139
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(127)	1,764
Other non-current assets	(180)
Deferred revenue	63,388	(3,790)
Accounts payable	402	361
Accrued expenses	(3,306)	3,950
Other non-current liabilities		1,000
Lease incentive benefit	321	515
Net cash provided by (used in) operating activities	8,461	(46,516)
Cash flow from investing activities
Purchases of property and equipment	(3,782)	(3,824)
Purchases of marketable securities	(274,381)	(49,856)
Proceeds from maturities of marketable securities	276,000	110,100
Net cash (used in) provided by investing activities	(2,163)	56,420
Cash flow from financing activities
Proceeds from the exercise of stock options	3,472	1,656
Proceeds from the purchase of common stock under ESPP	842
Net cash provided by financing activities	4,314	1,656
Net increase in cash, cash equivalents, and restricted cash	10,612	11,560
Cash, cash equivalents, and restricted cash beginning of period	32,265	37,376
Cash, cash equivalents, and restricted cash end of period	42,877	$ 48,936
Supplemental disclosure of cash and non-cash activities
Impact of adopting new accounting standards	$ 20,050