Note 9 - Net Loss Per Share Attributable to Common Stockholders - Anti-dilutive Securities (Details) - shares shares in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Potentially dilutive common shares (in shares)		35,838	31,111
Unvested Stock [Member]
Potentially dilutive common shares (in shares)	[1]	1,350	1,160
Convertible Preferred Stocks [Member]
Potentially dilutive common shares (in shares)		20,337	15,905
Common Stock Warrants [Member]
Potentially dilutive common shares (in shares)		2,069	1,964
IPO Warrants [Member]
Potentially dilutive common shares (in shares)	[2]	12,082	12,082

[1] Does not include 2.6 million shares for the three months ended March 31, 2021 and 2.8 million shares for the three months ended March 31, 2020, of unvested stock because the performance conditions had not yet been satisfied as of March 31, 2021 and 2020, respectively.

[2] The IPO Warrants were issued in connection with our initial public offering in 2015. The IPO Warrants are exercisable at a price of $11.50 per share and expire on July 24, 2022. The Company may redeem the IPO Warrants at a price of $0.01 per IPO Warrant upon 30 days’ notice only if the last sale price of our common stock is at least $17.50 per share for any 20 trading days within a 30-trading day period. If the Company redeems the IPO Warrants in this manner, the Company will have the option to do so on a cashless basis with the issuance of an economically equivalent number of shares of Company common stock.