Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flow from operating activities
Net loss	$ (19,926)	$ (16,648)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	2,322	2,165
Depreciation	456	317
Amortization of premiums and discounts on marketable securities	(309)	92
In-kind research and development expenses	176	44
Other non-cash items	(411)	855
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	305	958
Other non-current assets	(180)	(1,490)
Deferred revenue	68,057	(1,464)
Accounts payable	352	280
Accrued expenses	(2,232)	(504)
Other non-current liabilities		1,000
Lease incentive benefit		101
Net cash provided by (used in) operating activities	48,610	(14,294)
Cash flow from investing activities
Purchases of property and equipment	(1,607)	(2,945)
Purchases of marketable securities	(9,991)
Proceeds from maturities of marketable securities	70,000	23,600
Net cash provided by investing activities	58,402	20,655
Cash flow from financing activities
Proceeds from the exercise of stock options	1,411	602
Proceeds from the purchase of common stock under ESPP	418
Net cash provided by financing activities	1,829	602
Net increase in cash, cash equivalents, and restricted cash	108,841	6,963
Cash, cash equivalents, and restricted cash beginning of period	32,265	37,376
Cash, cash equivalents, and restricted cash end of period	141,106	44,339
Supplemental disclosure of cash and non-cash activities
Capital expenditures incurred but not yet paid	25	$ 387
Impact of adopting new accounting standards	$ 20,050