CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 189,156	$ 33,919	$ 359,743	$ (79,444)
Available-for-sale securities:
Unrealized holding gains (losses), net of tax effect	490	(404)	(354)	(1,729)
Reclassification of realized transactions, net of tax effect	0	7	0	89
Defined benefit plans:
Actuarial gains (losses) related to defined benefit pension plans, net of tax benefit of $2,311 $742, $1,385 and $3,745, respectively	618	(1,525)	(3,496)	(8,330)
Reclassification of realized transactions, net of tax effect	213	0	416	0
Change in net foreign currency translation adjustment	0	454	0	(2,569)
Other comprehensive income (loss), net of tax effect	1,321	(1,468)	(3,434)	(12,539)
Comprehensive income (loss)	$ 190,477	$ 32,451	$ 356,309	$ (91,983)