PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components of net periodic benefit costs:
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements			$ (2,863)	$ 0
Pension Plan [Member]
Components of net periodic benefit costs:
Service cost	$ 2,726	$ 3,257	5,494	6,513
Interest cost	22,755	21,950	45,432	43,902
Expected return on plan assets	(40,855)	(36,913)	81,709	73,826
Amortization of prior service costs	(1,059)	(1,322)	(2,205)	(2,644)
Defined Benefit Plan, Amortization of Gains (Losses)	2,468	0	(4,990)	0
Net periodic benefit cost	(13,965)	(13,028)	(25,135)	(26,055)
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	0	0
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments			(2,863)	0
Other postretirement
Components of net periodic benefit costs:
Service cost	306	717	632	1,434
Interest cost	2,056	3,082	4,126	6,164
Amortization of prior service costs	(1,451)	(1,132)	(2,796)	(2,264)
Defined Benefit Plan, Amortization of Gains (Losses)	(1,657)	0	(3,301)	0
Net periodic benefit cost	$ (746)	$ 2,667	$ (1,339)	$ 5,334