CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 656,168	$ 479,853
Short-term investments	1,215,944	665,567
Accounts receivable, net of allowance of $169 and $153 at March 31, 2021 and December 31, 2020, respectively	11,633	12,913
Prepaid expenses	25,256	12,776
Other current assets	26,625	11,846
Total current assets	1,935,626	1,182,955
Long-term investments	711,224	523,628
Textbook library, net	24,000	34,149
Property and equipment, net	134,093	125,807
Goodwill	290,601	285,214
Intangible assets, net	49,798	51,249
Right of use assets	22,617	24,226
Other assets	25,602	24,030
Total assets	3,193,561	2,251,258
Current liabilities
Accounts payable	14,902	8,547
Deferred revenue	48,608	32,620
Accrued liabilities	78,089	68,565
Current portion of convertible senior notes, net	109,494	0
Total current liabilities	251,093	109,732
Long-term liabilities
Long-term portion of convertible senior notes, net	1,673,946	1,506,922
Long-term operating lease liabilities	17,551	19,264
Other long-term liabilities	7,628	5,705
Total long-term liabilities	1,699,125	1,531,891
Total liabilities	1,950,218	1,641,623
Commitments and contingencies
Stockholders' equity:
Preferred stock, 0.001 par value – 10,000,000 shares authorized, no shares issued and outstanding	0	0
Common stock, 0.001 par value 400,000,000 shares authorized; 141,317,066 and 129,343,524 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	141	129
Additional paid-in capital	1,645,352	1,030,577
Accumulated other comprehensive (loss) income	(1,238)	1,530
Accumulated deficit	(400,912)	(422,601)
Total stockholders' equity	1,243,343	609,635
Total liabilities and stockholders' equity	$ 3,193,561	$ 2,251,258