8. STOCK- BASED COMPENSATION - Weighted average assumptions (Details) - $ / shares	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stock-based compensation
Weighted average fair value per underlying share of options granted (in dollars per share)	$ 2.53	$ 1.57	$ 2.54	$ 1.50
Employee stock options
Stock-based compensation
Risk-free interest rate (as a percent)	1.84%	1.12%
Risk-free interest rate, low end of range (as a percent)			1.84%	1.12%
Risk-free interest rate, high end of range (as a percent)			1.91%	1.26%
Expected life	5 years	5 years	5 years	5 years
Volatility (as a percent)	36.50%	33.90%	36.50%
Volatility, low end of range (as a percent)				33.90%
Volatility, high end of range (as a percent)				34.80%
Employee stock purchase plan options
Stock-based compensation
Risk-free interest rate (as a percent)			1.04%	0.38%
Expected life			6 months	6 months
Volatility (as a percent)			51.10%	39.60%