PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of net periodic benefit costs:
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments	$ (523)	$ 0	$ (523)	$ 0
Pension Plan [Member]
Components of net periodic benefit costs:
Service cost	1,124	1,635	2,244	3,284
Interest cost	18,801	18,161	37,589	36,350
Expected return on plan assets	(38,084)	(39,002)	(76,132)	(78,059)
Amortization of prior service costs	(710)	(445)	(1,421)	(891)
Defined Benefit Plan, Amortization of Gains (Losses)	3,477	3,029	6,925	6,060
Net periodic benefit cost	(14,869)	(16,622)	(30,272)	(33,256)
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments	(523)	0	(523)	0
Other postretirement
Components of net periodic benefit costs:
Service cost	102	179	203	358
Interest cost	1,219	1,247	2,438	2,494
Amortization of prior service costs	(2,328)	(3,366)	(4,656)	(6,732)
Defined Benefit Plan, Amortization of Gains (Losses)	(1,775)	(1,647)	(3,549)	(3,294)
Net periodic benefit cost	$ (2,782)	$ (3,587)	$ (5,564)	$ (7,174)