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]
Balance, beginning of period at Mar. 31, 2020	$ 4,220,720	$ 10,497	$ 453,819	$ 34,652	$ 4,399,402	$ (525,653)	$ (151,997)
Cosolidated statement of change in equity
Purchase of ESOP shares	(2,880)	0	0	0	(2,880)	0	0
Foreign currency translation	(5,193)	0	0	(5,193)	0	0	0
Unrealized net gain (loss) on investments, net of tax	54,900	0	0	54,900	0	0	0
Change in fair value of cash flow hedges, net of tax	(471)	0	0	(471)	0	0	0
Amounts reclassified into earnings on hedging activities, net	2,022	0	0	2,022	0	0	0
Net earnings	537,076	0	0	0	537,076	0	0
Common stock dividends	(49,019)	0	0	0	(49,019)	0	0
Net activity	536,435	0	0	51,258	485,177	0	0
Balance, end of period at Dec. 31, 2020	4,757,155	10,497	453,819	85,910	4,884,579	(525,653)	(151,997)
Balance, beginning of period at Sep. 30, 2020	4,605,631	10,497	453,819	78,156	4,740,809	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	(2,566)	0	0	(2,566)	0	0	0
Unrealized net gain (loss) on investments, net of tax	9,560	0	0	9,560	0	0	0
Change in fair value of cash flow hedges, net of tax	26	0	0	26	0	0	0
Amounts reclassified into earnings on hedging activities, net	734	0	0	734	0	0	0
Net earnings	182,985	0	0	0	182,985	0	0
Common stock dividends	(39,215)	0	0	0	(39,215)	0	0
Net activity	151,524	0	0	7,754	143,770	0	0
Balance, end of period at Dec. 31, 2020	4,757,155	10,497	453,819	85,910	4,884,579	(525,653)	(151,997)
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	(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, net of tax	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
Common stock dividends	(29,412)	0	0	0	(29,412)	0	0
Net activity	963,534	0	0	(43,593)	1,007,127	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)
Balance, beginning of period at Sep. 30, 2021	5,554,697	10,497	453,819	74,207	5,693,824	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	112	0	0	112	0	0	0
Unrealized net gain (loss) on investments, net of tax	(11,992)	0	0	(11,992)	0	0	0
Change in fair value of cash flow hedges, net of tax	178	0	0	178	0	0	0
Amounts reclassified into earnings on hedging activities, net	759	0	0	759	0	0	0
Net earnings	281,466	0	0	0	281,466	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	260,719	0	0	(10,943)	271,662	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)