CONDENSED CONSOLIDATED BALANCE SHEETS (unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 359,101	$ 387,520
Short-term investments	404,794	381,074
Accounts receivable, net of allowance of $160 and $56 at March 31, 2020 and December 31, 2019, respectively	6,493	11,529
Prepaid expenses	17,852	10,538
Other current assets	19,528	16,606
Total current assets	807,768	807,267
Long-term investments	273,114	310,483
Textbook library, net	31,673	0
Property and equipment, net	97,889	87,359
Goodwill	214,323	214,513
Intangible assets, net	32,075	34,667
Right of use assets	14,608	15,931
Other assets	23,554	18,778
Total assets	1,495,004	1,488,998
Current liabilities
Accounts payable	13,605	7,362
Deferred revenue	35,753	18,780
Current operating lease liabilities	5,234	5,283
Accrued liabilities	47,241	39,964
Total current liabilities	101,833	71,389
Long-term liabilities
Convertible senior notes, net	913,249	900,303
Long-term operating lease liabilities	13,012	14,513
Other long-term liabilities	3,983	3,964
Total long-term liabilities	930,244	918,780
Total liabilities	1,032,077	990,169
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; 123,542,833 and 121,583,501 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	124	122
Additional paid-in capital	890,258	916,095
Accumulated other comprehensive loss	(5,362)	(1,096)
Accumulated deficit	(422,093)	(416,292)
Total stockholders' equity	462,927	498,829
Total liabilities and stockholders' equity	$ 1,495,004	$ 1,488,998