Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 229,652	$ 120,320
Marketable securities	461,848	459,256
Prepaid expenses and other current assets	11,979	13,832
Total current assets	703,479	593,408
Long-term investment—restricted	2,125	1,510
Property and equipment, net	6,693	6,333
Operating lease assets	20,630	21,158
Total assets	732,927	622,409
Current liabilities:
Accounts payable	11,337	19,575
Accrued expenses and other current liabilities	32,234	38,716
Operating lease liabilities	1,501	1,747
Total current liabilities	45,072	60,038
Operating lease liabilities, net of current portion	15,595	15,904
Total liabilities	60,667	75,942
Commitments and contingencies (Note 6)
Stockholders' equity:
Preferred stock, $0.01 par value per share; 5,000,000 shares authorized; no shares issued or outstanding	0	0
Common stock, $0.01 par value per share; 125,000,000 shares authorized; 55,681,027 shares and 51,617,639 shares issued and outstanding as of March 31, 2020 and December 31, 2019, respectively	557	516
Additional paid-in capital	1,231,726	1,033,819
Accumulated other comprehensive income (loss)	763	111
Accumulated deficit	(560,786)	(487,979)
Total stockholders' equity	672,260	546,467
Total liabilities and stockholders' equity	$ 732,927	$ 622,409