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]	Nonvoting Common Stock [Member] 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
Common stock dividend		0	(176)	0	0	0	0		(176)
Foreign currency translation	757	0	0	757	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	304,620	0	0	304,620	0	0	0		0
Change in fair value of cash flow hedges	6,532	0	0	6,532	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	101	0	0	101	0	0	0		0
Net Income (Loss)	887,063	0	0	0	887,063	0	0	$ 887,063	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0		0
Series N Non-Voting Common Stock dividends paid	(7,059)	0	0	0	(7,059)	0	0		0
Net activity	561,652	0	176	298,744	860,396	0	0		176
Balance, end of period at Dec. 31, 2022	6,455,727	10,497	453,643	303,736	6,972,797	525,653	151,997		176
Balance, beginning of period at Sep. 30, 2022	6,324,173	10,497	453,819	243,497	6,781,004	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	59,794	0	0	59,794	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 Income (Loss)	198,852	0	0	0	198,852	0	0		0
Series N Non-Voting Common Stock dividends paid	(7,059)	0	0	0	(7,059)	0	0		0
Net activity	131,554	0	(176)	60,239	191,793	0	0		176
Balance, end of period at Dec. 31, 2022	6,455,727	10,497	453,643	303,736	6,972,797	525,653	151,997		176
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	2,888	0	0	2,888	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	25,738	0	0	25,738	0	0	0		0
Change in fair value of cash flow hedges	2,823	0	0	2,823	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	2,992	0	0	2,992	0	0	0		0
Net Income (Loss)	629,570	0	0	0	629,570	0	0	$ 629,570	0
Series N Non-Voting Common Stock dividends paid	(22,941)	0	0	0	(22,941)	0	0		0
Net activity	583,610	0	0	23,019	606,629	0	0		0
Balance, end of period at Dec. 31, 2023	7,087,801	10,497	453,643	308,642	7,609,777	525,653	151,997		176
Balance, beginning of period at Sep. 30, 2023	7,030,378	10,497	453,643	275,664	7,519,376	525,653	151,997		176
Cosolidated statement of change in equity
Foreign currency translation	5,268	0	0	5,268	0	0	0		0
Unrealized net gain (loss) on investments, net of tax	32,785	0	0	32,785	0	0	0		0
Change in fair value of cash flow hedges	4,352	0	0	4,352	0	0	0		0
Amounts reclassified into earnings on hedging activities, net	1,109	0	0	1,109	0	0	0		0
Net Income (Loss)	99,224	0	0	0	99,224	0	0		0
Common stock dividends	(8,823)	0	0	0	(8,823)	0	0		0
Net activity	57,423	0	0	32,978	90,401	0	0		0
Balance, end of period at Dec. 31, 2023	$ 7,087,801	$ 10,497	$ 453,643	$ 308,642	$ 7,609,777	$ 525,653	$ 151,997		$ 176