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]	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	197	0	0	197	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(136,382)	0	0	(136,382)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	128	0	0	128	0	0	0	0
Amounts reclassified into earnings on hedging activities	427	0	0	427	0	0	0	0
Net earnings	338,345	0	0	0	338,345	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	192,911	0	0	(135,630)	328,541	0	0	0
Balance, end of period at Jun. 30, 2022	6,086,986	10,497	453,819	(140,622)	6,440,942	(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	469	0	0	469	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	20,344	0	0	20,344	0	0	0	0
Change in fair value of cash flow hedges, net of tax	3,842	0	0	3,842	0	0	0	0
Amounts reclassified into earnings on hedging activities	(868)	0	0	(868)	0	0	0	0
Net earnings	256,838	0	0	0	256,838	0	0	0
Series N Non-Voting Common Stock Dividends	(7,059)	0	0	0	(7,059)	0	0	0
Net activity	273,566	0	0	23,787	249,779	0	0	0
Balance, end of period at Jun. 30, 2023	$ 6,777,757	$ 10,497	$ 453,643	$ (261,836)	$ 7,252,927	$ (525,653)	$ (151,997)	$ 176