Note 6 - Stockholders' Equity - Options Outstanding and Exercisable, by Exercise Range (Details) shares in Thousands	9 Months Ended
	Dec. 31, 2017 $ / shares shares
Lower range of exercise price (in dollars per share)	$ 1.35
Upper range of exercise price (in dollars per share)	$ 11.40
Options outstanding, number of shares (in shares) | shares	4,286
Options outstanding, weighted-average remaining contractual life (Year)	7 years 332 days
Options vested, number of shares (in shares) | shares	2,110
Options vested, weighted-average exercise price per share (in dollars per share)	$ 3.39
Range 1 [Member]
Lower range of exercise price (in dollars per share)	1.35
Upper range of exercise price (in dollars per share)	$ 1.35
Options outstanding, number of shares (in shares) | shares	92
Options outstanding, weighted-average remaining contractual life (Year)	8 years 142 days
Options vested, number of shares (in shares) | shares	38
Options vested, weighted-average exercise price per share (in dollars per share)	$ 1.35
Range 2 [Member]
Lower range of exercise price (in dollars per share)	1.64
Upper range of exercise price (in dollars per share)	$ 1.64
Options outstanding, number of shares (in shares) | shares	1,925
Options outstanding, weighted-average remaining contractual life (Year)	8 years 54 days
Options vested, number of shares (in shares) | shares	873
Options vested, weighted-average exercise price per share (in dollars per share)	$ 1.64
Range 3 [Member]
Lower range of exercise price (in dollars per share)	1.75
Upper range of exercise price (in dollars per share)	$ 2
Options outstanding, number of shares (in shares) | shares	560
Options outstanding, weighted-average remaining contractual life (Year)	8 years 273 days
Options vested, number of shares (in shares) | shares	192
Options vested, weighted-average exercise price per share (in dollars per share)	$ 1.86
Range 4 [Member]
Lower range of exercise price (in dollars per share)	2.59
Upper range of exercise price (in dollars per share)	$ 3.99
Options outstanding, number of shares (in shares) | shares	448
Options outstanding, weighted-average remaining contractual life (Year)	8 years 94 days
Options vested, number of shares (in shares) | shares	206
Options vested, weighted-average exercise price per share (in dollars per share)	$ 3.44
Range 5 [Member]
Lower range of exercise price (in dollars per share)	4.32
Upper range of exercise price (in dollars per share)	$ 4.32
Options outstanding, number of shares (in shares) | shares	627
Options outstanding, weighted-average remaining contractual life (Year)	7 years 211 days
Options vested, number of shares (in shares) | shares	378
Options vested, weighted-average exercise price per share (in dollars per share)	$ 4.32
Range 6 [Member]
Lower range of exercise price (in dollars per share)	5.18
Upper range of exercise price (in dollars per share)	$ 6.30
Options outstanding, number of shares (in shares) | shares	369
Options outstanding, weighted-average remaining contractual life (Year)	6 years 266 days
Options vested, number of shares (in shares) | shares	287
Options vested, weighted-average exercise price per share (in dollars per share)	$ 5.95
Range 7 [Member]
Lower range of exercise price (in dollars per share)	6.83
Upper range of exercise price (in dollars per share)	$ 6.61
Options outstanding, number of shares (in shares) | shares	205
Options outstanding, weighted-average remaining contractual life (Year)	6 years 73 days
Options vested, number of shares (in shares) | shares	176
Range 8 [Member]
Lower range of exercise price (in dollars per share)	$ 7.20
Upper range of exercise price (in dollars per share)	$ 6.83
Options outstanding, number of shares (in shares) | shares	50
Options outstanding, weighted-average remaining contractual life (Year)	6 years 51 days
Options vested, number of shares (in shares) | shares	50
Options vested, weighted-average exercise price per share (in dollars per share)	$ 6.83
Range 9 [Member]
Lower range of exercise price (in dollars per share)	11.40
Upper range of exercise price (in dollars per share)	$ 7.20
Options outstanding, number of shares (in shares) | shares	5
Options outstanding, weighted-average remaining contractual life (Year)	1 year 58 days
Options vested, number of shares (in shares) | shares	5
Options vested, weighted-average exercise price per share (in dollars per share)	$ 7.20
Range 10 [Member]
Lower range of exercise price (in dollars per share)	18.90
Upper range of exercise price (in dollars per share)	$ 11.40
Options outstanding, number of shares (in shares) | shares	5
Options outstanding, weighted-average remaining contractual life (Year)	233 days
Options vested, number of shares (in shares) | shares	5
Options vested, weighted-average exercise price per share (in dollars per share)	$ 11.40