Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 112,026	$ 132,037	$ 343,737	$ 328,954
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in unrealized (loss) gain on derivatives designated as cash flow hedges	4,945		(15,034)
Change in unrealized (loss) gain on derivatives designated as cash flow hedges		(7,058)		(4,100)
Change in postretirement plan costs	17	447	67	1,278
Total other comprehensive income (loss), net of tax	4,962	(6,611)	(14,967)	(2,822)
Comprehensive income	$ 116,988	$ 125,426	$ 328,770	$ 326,132