Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Less: Treasury Common Stock [Member]	Less: Treasury Preferred Stock [Member]
Balance, beginning of period at Mar. 31, 2021	$ 4,851,882	$ 10,497	$ 453,819	$ 106,857	$ 4,958,359	$ (525,653)	$ (151,997)
Cosolidated statement of change in equity
Foreign currency translation	(3,392)	0	0	(3,392)	0	0	0
Unrealized net gain (loss) on investments, net of tax	(73,027)	0	0	(73,027)	0	0	0
Change in fair value of cash flow hedges, net of tax	(51)	0	0	(51)	0	0	0
Amounts reclassified into earnings on hedging activities	745	0	0	745	0	0	0
Net earnings	345,175	0	0	0	345,175	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	259,646	0	0	(75,725)	335,371	0	0
Balance, end of period at Jun. 30, 2021	5,111,528	10,497	453,819	31,132	5,293,730	(525,653)	(151,997)
Balance, beginning of period at Mar. 31, 2022	5,885,283	10,497	453,819	46,384	6,052,233	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	197	0	0	197	0	0	0
Unrealized net gain (loss) on investments, net of tax	(136,382)	0	0	(136,382)	0	0	0
Change in fair value of cash flow hedges, net of tax	128	0	0	128	0	0	0
Amounts reclassified into earnings on hedging activities	427	0	0	427	0	0	0
Net earnings	334,002	0	0	0	334,002	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	188,568	0	0	(135,630)	324,198	0	0
Balance, end of period at Jun. 30, 2022	$ 6,073,851	$ 10,497	$ 453,819	$ (89,246)	$ 6,376,431	$ (525,653)	$ (151,997)