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]	Less: Unearned Employee Stock Ownership Plan Shares [Member]	Nonvoting Common Stock [Member]
Balance, beginning of period at Mar. 31, 2022	$ 5,894,075	$ 10,497	$ 453,819	$ (4,992)	$ 6,112,401	$ (525,653)	$ (151,997)		$ 0
Cosolidated statement of change in equity
Foreign currency translation	(542)	0	0	(542)	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(244,826)	0	0	(244,826)	0	0	0		0
Change in fair value of cash flow hedges	6,417	0	0	6,417	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	446	0	0	446	0	0	0		0
Net earnings	688,211	0	0	0	688,211	0	0		0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0		0
Net activity	430,098	0	0	(238,505)	668,603	0	0		0
Balance, end of period at Sep. 30, 2022	6,324,173	10,497	453,819	(243,497)	6,781,004	(525,653)	(151,997)		0
Balance, beginning of period at Jun. 30, 2022	6,086,986	10,497	453,819	(140,622)	6,440,942	(525,653)	(151,997)		0
Cosolidated statement of change in equity
Foreign currency translation	(739)	0	0	(739)	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(108,444)	0	0	(108,444)	0	0	0		0
Change in fair value of cash flow hedges	6,289	0	0	6,289	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	19	0	0	19	0	0	0		0
Net earnings	349,866	0	0	0	349,866	0	0		0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0		0
Net activity	237,187	0	0	(102,875)	340,062	0	0		0
Balance, end of period at Sep. 30, 2022	6,324,173	10,497	453,819	(243,497)	6,781,004	(525,653)	(151,997)		0
Balance, beginning of period at Mar. 31, 2023	6,504,191	10,497	453,643	(285,623)	7,003,148	(525,653)	(151,997)		176
Cosolidated statement of change in equity
Foreign currency translation	(2,380)	0	0	(2,380)	0	0	0	$ 0	0
Unrealized net gain (loss) on investments, net of tax	7,047	0	0	7,047	0	0	0	0	0
Change in fair value of cash flow hedges	7,175	0	0	7,175	0	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	(1,883)	0	0	(1,883)	0	0	0	$ 0	0
Net earnings	530,346	0	0	0	530,346	0	0		0
Common stock dividends	(14,118)	0	0	0	(14,118)	0	0		0
Net activity	526,187	0	0	9,959	516,228	0	0		0
Balance, end of period at Sep. 30, 2023	7,030,378	10,497	453,643	(275,664)	7,519,376	(525,653)	(151,997)		176
Balance, beginning of period at Jun. 30, 2023	6,777,757	10,497	453,643	(261,836)	7,252,927	(525,653)	(151,997)		176
Cosolidated statement of change in equity
Foreign currency translation	(2,849)	0	0	(2,849)	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(13,297)	0	0	(13,297)	0	0	0		0
Change in fair value of cash flow hedges	3,333	0	0	3,333	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	(1,015)	0	0	(1,015)	0	0	0		0
Net earnings	273,508	0	0	0	273,508	0	0		0
Common stock dividends	(7,059)	0	0	0	(7,059)	0	0		0
Net activity	252,621	0	0	(13,828)	266,449	0	0		0
Balance, end of period at Sep. 30, 2023	$ 7,030,378	$ 10,497	$ 453,643	$ (275,664)	$ 7,519,376	$ (525,653)	$ (151,997)		$ 176