Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Comprehensive income (loss) (pretax):
Net earnings	$ 378,124	$ 477,529	$ 637,613
Comprehensive income (loss) (net of tax):
Net earnings	442,048	370,857	790,583
Other comprehensive income (loss):
Foreign currency translation (pretax)	9,377	(1,759)	14,652
Foreign currency translation (tax effect)	0	0	0
Foreign currency translation (net of tax)	9,377	(1,759)	14,652
Unrealized gain (loss) on investments (pretax)	124,566	(76,124)	30,929
Unrealized gain (loss) on investments (tax effect)	(26,623)	16,356	(10,825)
Unrealized gain (loss) on investments (net of tax)	97,943		20,104
Change in fair value of cash flow hedges (pretax)	(8,352)	598	4,445
Change in fair value of cash flow hedges (tax effect)	2,051	(147)	(1,363)
Change in fair value of cash flow hedges (net of tax)			3,082
Amounts reclassifed into earnings on hedging	(3)	35	0
Amounts reclassified into earnings on hedging, tax			0
Amounts reclassified into earnings on hedging, net	(401)	789	3,893
Postretirement benefit obligation gain (loss) (pretax)	441	(1,359)	288
Postretirement benefit obligation gain (loss) (tax effect)	(108)	334	(253)
Postretirement benefit obligation gain (loss) (net of tax)	333	(1,025)	35
Total other comprehensive income loss	101,350	(62,075)	37,873
Total comprehensive income, net	$ 543,398	$ 308,782	$ 828,456