Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Comprehensive income (loss) (pretax):
Net earnings	$ 637,613	$ 628,358	$ 768,911
Comprehensive income (loss) (tax effect):
Net earnings	152,970	(229,934)	(279,910)
Comprehensive income (loss) (net of tax):
Net earnings	790,583	398,424	489,001
Other comprehensive income (loss):
Foreign currency translation (pretax)	14,652	(5,862)	(4,473)
Foreign currency translation (tax effect)
Foreign currency translation (net of tax)	14,652	(5,862)	(4,473)
Unrealized gain (loss) on investments (pretax)	30,929	13,822	(41,639)
Unrealized gain (loss) on investments (tax effect)	(10,825)	(4,838)	14,573
Unrealized gain (loss) on investments (net of tax)	20,104	8,984	(27,066)
Change in fair value of cash flow hedges (pretax)	4,445	9,916	9,721
Change in fair value of cash flow hedges (tax effect)	(1,363)	(3,767)	(3,694)
Change in fair value of cash flow hedges (net of tax)	3,082	6,149	6,027
Postretirement benefit obligation gain (loss) (pretax)	288	28	(1,029)
Postretirement benefit obligation gain (loss) (tax effect)	(253)	(10)	381
Postretirement benefit obligation gain (loss) (net of tax)	35	18	(648)
Total comprehensive income (loss) (pretax)	687,927	646,262	731,491
Total comprehensive income (loss) (tax effect)	140,529	(238,549)	(268,650)
Total comprehensive income (loss) (net of tax)	$ 828,456	$ 407,713	$ 462,841