Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (84,178)	$ (60,814)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	17,003	15,128
Amortization of investment premiums and discounts	607	2,811
Depreciation expense	678	210
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,125)	(602)
Other assets	2	6
Accounts payable	(1)	3,018
Accrued compensation	1,574	532
Accrued research and development expenses	1,402	(3)
Other accrued liabilities	1,104	433
Long-term liabilities	370	420
Net cash used in operating activities	(62,564)	(38,861)
Investing activities
Purchases of short-term investments	(152,837)	(252,279)
Maturities of short-term investments	162,094	127,749
Sales of short-term investments	58,217	187,508
Purchases of property and equipment	(10,535)	(2,025)
Restricted cash	(1,200)	0
Net cash provided by investing activities	55,739	60,953
Financing activities
Proceeds from issuance of common stock from "at-the-market" facility, net	19,206	0
Taxes paid related to net share settlement of restricted stock units	(351)	(75)
Proceeds from employee stock awards	495	250
Net cash provided by financing activities	19,350	175
Increase in cash and cash equivalents	12,525	22,267
Cash and cash equivalents at beginning of period	47,968	23,746
Cash and cash equivalents at end of period	60,493	46,013
Non-cash investing and financing activities
Capitalized lease obligations	6,215	0
Property and equipment purchases included in liabilities	3,064	129
Interest capitalized during construction period for build-to-suit lease transaction	133	0
Issuance of common stock upon vesting of stock awards	0	60
Change in long-term liabilities related to non-vested stock awards	0	(60)
Supplemental cash flow disclosure
Cash paid for taxes	$ 0	$ 10