CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Allowance for accounts receivable	$ 2,757	$ 1,919
Prepaid expenses and other current assets	16,910	15,357
Current liabilities:
Accrued expenses	48,380	55,507
Contract liabilities	58,529	58,158
Non-current liabilities	$ 19,600	$ 13,284
Stockholders' equity:
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares issued (in shares)	79,703,342	76,829,926
Common stock, shares outstanding (in shares)	72,938,546	70,065,130
Treasury stock, at cost (in shares)	6,764,796	6,764,796
Investor
Current assets:
Accounts receivable, net	$ 4,045	$ 2,698
Prepaid expenses and other current assets	1,496	1,180
Current liabilities:
Accounts payable	2,817	2,510
Accrued expenses	835	6,902
Contract liabilities	3,538	1,519
Non-current liabilities	$ 6,120	$ 0