FAIR VALUE OF FINANCIAL INSTRUMENTS: (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Fair value of assets and liabilities
Assets of non-qualified retirement plan	$ 15,223	$ 13,551
Other current assets	23,196	$ 20,173
Fair value measurements on recurring basis | Total
Fair value of assets and liabilities
Other current assets	15,223
Total assets	15,223
Fair value measurements on recurring basis | Total | Level 1
Fair value of assets and liabilities
Assets of non-qualified retirement plan	15,223
Total assets	15,223
Fair value measurements on recurring basis | Total | Level 2
Fair value of assets and liabilities
Other current assets	0
Total assets	0
Fair value measurements on recurring basis | Total | Level 3
Fair value of assets and liabilities
Other current assets	0
Total assets	$ 0