8. STOCK- BASED COMPENSATION - Weighted average assumptions (Details) - $ / shares	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Stock-based compensation
Weighted average fair value per underlying share of options granted (in dollars per share)	$ 2.76	$ 2.58
Employee stock options
Stock-based compensation
Risk-free interest rate, high end of range (as a percent)	2.78%	1.91%
Expected life	5 years	5 years
Volatility, high end of range (as a percent)	35.60%	36.50%
Employee stock purchase plan options
Stock-based compensation
Risk-free interest rate, high end of range (as a percent)	2.09%	1.04%
Expected life	6 months	6 months
Volatility, high end of range (as a percent)	37.70%	51.10%