Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 482,603	$ 418,529	$ 301,692
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in unrealized gain (loss) on derivatives designated as cash flow hedges	(39,752)
Change in unrealized gain (loss) on derivatives designated as cash flow hedges		(7,971)	4,993
Change in postretirement plan costs	4,941	11,887	(171)
Total other comprehensive (loss) income, net of tax	(34,811)	3,916	4,822
Comprehensive income	$ 447,792	$ 422,445	$ 306,514