Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash Flows from Operating Activities
Net loss	$ (69,595)	$ (36,950)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,412	5,846
Noncash lease expense	1,870	1,424
Amortization of premiums on investments	(50)	(1,232)
Depreciation and amortization	252	271
Gain on settlement of dispute		(336)
Changes in assets and liabilities:
Prepaid expenses, other assets, and long-term assets	(1,316)	1,592
Operating lease liabilities (Right-of-use assets)	(2,461)	(942)
Accounts payable	(7,768)	4,583
Accrued expenses and other liabilities	4,364	(3,346)
Net cash used in operating activities	(65,292)	(29,090)
Cash Flows from Investing Activities
Maturities of short-term investments	113,665	91,042
Purchase of short-term investments	(12,923)	(89,185)
Purchase of property and equipment	(637)	(824)
Net cash provided by investing activities	100,105	1,033
Cash Flows from Financing Activities
Tax payments related to shares withheld for vested restricted stock awards	(118)	(71)
Proceeds from the issuance of common stock upon exercise of options	3,951	69
Net cash provided by / (used in) financing activities	3,833	(2)
Net increase / (decrease) in cash, cash equivalents and restricted cash	38,646	(28,059)
Cash, Cash Equivalents, and Restricted Cash, Beginning of Period	19,419	82,152
Cash, Cash Equivalents, and Restricted Cash, End of Period (Note 2)	58,065	54,093
Supplemental disclosure of non-cash investing and financing activities:
Net unrealized gain on short-term investments	692	180
Acquisitions of property and equipment included in accounts payable and accrued expenses	$ 4,686	$ 268