8. STOCK- BASED COMPENSATION - Options outstanding by exercise price (Details) - $ / shares	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018
Options outstanding by exercise price
Exercise Price (in dollars per share)	$ 5.53	$ 5.45
Number of Shares Underlying Options Outstanding (in shares)	7,613,779
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 5.53
Options Outstanding, Weighted Average Remaining Contractual Life	5 years 6 months 11 days
Options Exercisable, Number Vested and Exercisable (in shares)	5,046,534
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 5.34
$2.43 - $3.60
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	2.43
Exercise Price, high end of range (in dollars per share)	$ 3.60
Number of Shares Underlying Options Outstanding (in shares)	794,891
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 3.36
Options Outstanding, Weighted Average Remaining Contractual Life	3 years 11 months 27 days
Options Exercisable, Number Vested and Exercisable (in shares)	608,537
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 3.33
$3.75 - $4.34
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	3.75
Exercise Price, high end of range (in dollars per share)	$ 4.34
Number of Shares Underlying Options Outstanding (in shares)	828,326
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 4.09
Options Outstanding, Weighted Average Remaining Contractual Life	2 years 8 months 1 day
Options Exercisable, Number Vested and Exercisable (in shares)	792,876
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 4.08
$4.43 - $4.98
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	4.43
Exercise Price, high end of range (in dollars per share)	$ 4.98
Number of Shares Underlying Options Outstanding (in shares)	1,052,821
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 4.88
Options Outstanding, Weighted Average Remaining Contractual Life	5 years 4 months 13 days
Options Exercisable, Number Vested and Exercisable (in shares)	629,617
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 4.83
$4.99 - $5.23
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	4.99
Exercise Price, high end of range (in dollars per share)	$ 5.23
Number of Shares Underlying Options Outstanding (in shares)	1,157,976
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 5.10
Options Outstanding, Weighted Average Remaining Contractual Life	7 years 2 months 16 days
Options Exercisable, Number Vested and Exercisable (in shares)	637,576
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 5.18
$5.28 - $5.76
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	5.28
Exercise Price, high end of range (in dollars per share)	$ 5.76
Number of Shares Underlying Options Outstanding (in shares)	762,790
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 5.55
Options Outstanding, Weighted Average Remaining Contractual Life	5 years 11 months 23 days
Options Exercisable, Number Vested and Exercisable (in shares)	619,466
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 5.61
$6.00 - $6.45
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	6.00
Exercise Price, high end of range (in dollars per share)	$ 6.45
Number of Shares Underlying Options Outstanding (in shares)	777,918
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.19
Options Outstanding, Weighted Average Remaining Contractual Life	5 years
Options Exercisable, Number Vested and Exercisable (in shares)	550,685
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.20
$6.54 - $6.86
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	6.54
Exercise Price, high end of range (in dollars per share)	$ 6.86
Number of Shares Underlying Options Outstanding (in shares)	1,114,076
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.74
Options Outstanding, Weighted Average Remaining Contractual Life	5 years 2 months 16 days
Options Exercisable, Number Vested and Exercisable (in shares)	874,185
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.70
$7.00 - $7.40
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	7.00
Exercise Price, high end of range (in dollars per share)	$ 7.40
Number of Shares Underlying Options Outstanding (in shares)	812,001
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 7.22
Options Outstanding, Weighted Average Remaining Contractual Life	7 years 8 months 1 day
Options Exercisable, Number Vested and Exercisable (in shares)	225,012
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 7.09
$7.70 - $8.09
Options outstanding by exercise price
Exercise Price, low end of range (in dollars per share)	7.70
Exercise Price, high end of range (in dollars per share)	$ 8.09
Number of Shares Underlying Options Outstanding (in shares)	204,400
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 7.95
Options Outstanding, Weighted Average Remaining Contractual Life	9 years 8 months 5 days
$9.20
Options outstanding by exercise price
Exercise Price (in dollars per share)	$ 9.20
Number of Shares Underlying Options Outstanding (in shares)	108,580
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 9.20
Options Outstanding, Weighted Average Remaining Contractual Life	2 years 7 months 2 days
Options Exercisable, Number Vested and Exercisable (in shares)	108,580
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 9.20