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, 2021	$ 4,851,882	$ 10,497	$ 453,819	$ 106,857	$ 4,958,359	$ (525,653)	$ (151,997)		$ 0
Cosolidated statement of change in equity
Foreign currency translation	(2,288)	0	0	(2,288)	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(43,637)	0	0	(43,637)	0	0	0		0
Change in fair value of cash flow hedges	71	0	0	71	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	2,261	0	0	2,261	0	0	0		0
Net earnings	1,036,539	0	0	0	1,036,539	0	0		0
Common stock dividends	(29,412)	0	0	0	(29,412)	0	0		0
Series N Non-Voting Common Stock dividends	0
Net activity	963,534	0	0	(43,593)	1,007,127	0	0		0
Balance, end of period at Dec. 31, 2021	5,815,416	10,497	453,819	63,264	5,965,486	(525,653)	(151,997)		0
Balance, beginning of period at Sep. 30, 2021	5,554,697	10,497	453,819	74,207	5,693,824	(525,653)	(151,997)		0
Cosolidated statement of change in equity
Foreign currency translation	112	0	0	112	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(11,992)	0	0	(11,992)	0	0	0		0
Change in fair value of cash flow hedges	178	0	0	178	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	759	0	0	759	0	0	0		0
Net earnings	281,466	0	0	0	281,466	0	0		0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0		0
Net activity	260,719	0	0	(10,943)	271,662	0	0		0
Balance, end of period at Dec. 31, 2021	5,815,416	10,497	453,819	63,264	5,965,486	(525,653)	(151,997)		0
Balance, beginning of period at Mar. 31, 2022	5,885,283	10,497	453,819	46,384	6,052,233	(525,653)	(151,997)		0
Cosolidated statement of change in equity
Common Stock Dividend	0	0	(176)	0	0	0	0		176
Foreign currency translation	(757)	0	0	(757)	0	0	0	$ 0	0
Unrealized net gain (loss) on investments, net of tax	(336,376)	0	0	(336,376)	0	0	0	0	0
Change in fair value of cash flow hedges	6,532	0	0	6,532	0	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	101	0	0	101	0	0	0	$ 0	0
Net earnings	885,261	0	0	0	885,261	0	0		0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0		0
Series N Non-Voting Common Stock dividends	(7,059)	0	0	0	(7,059)	0	0		0
Net activity	528,094	0	(176)	(330,500)	858,594	0	0		176
Balance, end of period at Dec. 31, 2022	6,413,377	10,497	453,643	(284,116)	6,910,827	(525,653)	(151,997)		176
Balance, beginning of period at Sep. 30, 2022	6,313,187	10,497	453,819	(192,121)	6,718,642	(525,653)	(151,997)		0
Cosolidated statement of change in equity
Common Stock Dividend	0	0	(176)	0	0	0	0		176
Foreign currency translation	(215)	0	0	(215)	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	(91,550)	0	0	(91,550)	0	0	0		0
Change in fair value of cash flow hedges	115	0	0	115	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	(345)	0	0	(345)	0	0	0		0
Net earnings	199,244	0	0	0	199,244	0	0		0
Series N Non-Voting Common Stock dividends	(7,059)	0	0	0	(7,059)	0	0		0
Net activity	100,190	0	(176)	(91,995)	192,185	0	0		176
Balance, end of period at Dec. 31, 2022	$ 6,413,377	$ 10,497	$ 453,643	$ (284,116)	$ 6,910,827	$ (525,653)	$ (151,997)		$ 176