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, 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	(2,400)	0	0	(2,400)	0	0	0
Unrealized net gain (loss) on investments, net of tax	(31,645)	0	0	(31,645)	0	0	0
Change in fair value of cash flow hedges	(107)	0	0	(107)	0	0	0
Amounts reclassified into earnings on hedging activities, net	1,502	0	0	1,502	0	0	0
Net earnings	755,073	0	0	0	755,073	0	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0
Net activity	702,815	0	0	(32,650)	735,465	0	0
Balance, end of period at Sep. 30, 2021	5,554,697	10,497	453,819	74,207	5,693,824	(525,653)	(151,997)
Balance, beginning of period at Jun. 30, 2021	5,111,528	10,497	453,819	31,132	5,293,730	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	992	0	0	992	0	0	0
Unrealized net gain (loss) on investments, net of tax	41,382	0	0	41,382	0	0	0
Change in fair value of cash flow hedges	(56)	0	0	(56)	0	0	0
Amounts reclassified into earnings on hedging activities, net	757	0	0	757	0	0	0
Net earnings	409,898	0	0	0	409,898	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	443,169	0	0	43,075	400,094	0	0
Balance, end of period at Sep. 30, 2021	5,554,697	10,497	453,819	74,207	5,693,824	(525,653)	(151,997)
Balance, beginning of period at Mar. 31, 2022	5,885,283	10,497	453,819	46,384	6,052,233	(525,653)	(151,997)
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	686,017	0	0	0	686,017	0	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0
Net activity	427,904	0	0	(238,505)	666,409	0	0
Balance, end of period at Sep. 30, 2022	6,313,187	10,497	453,819	(192,121)	6,718,642	(525,653)	(151,997)
Balance, beginning of period at Jun. 30, 2022	6,073,851	10,497	453,819	(89,246)	6,376,431	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	(739)	0	0	(739)	0	0	0
Unrealized net gain (loss) on investments, net of tax	(108,444)	0	0	(108,444)	0	0	0
Change in fair value of cash flow hedges	6,289	0	0	6,289	0	0	0
Amounts reclassified into earnings on hedging activities, net	19	0	0	19	0	0	0
Net earnings	352,015	0	0	0	352,015	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	239,336	0	0	(102,875)	342,211	0	0
Balance, end of period at Sep. 30, 2022	$ 6,313,187	$ 10,497	$ 453,819	$ (192,121)	$ 6,718,642	$ (525,653)	$ (151,997)