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
Adjustment for adoption of ASU 2016-13	(2,880)	0	0	0	(2,880)	0	0
Foreign currency translation	(2,627)	0	0	(2,627)	0	0	0
Unrealized net gain (loss) on investments, net of tax	45,340	0	0	45,340	0	0	0
Change in fair value of cash flow hedges, net of tax	(497)	0	0	(497)	0	0	0
Amounts reclassified into earnings on hedging activities, net	1,288	0	0	1,288	0	0	0
Net earnings	354,091	0	0	0	354,091	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	384,911	0	0	43,504	341,407	0	0
Balance, end of period at Sep. 30, 2020	4,605,631	10,497	453,819	78,156	4,740,809	(525,653)	(151,997)
Balance, beginning of period at Jun. 30, 2020	4,257,182	10,497	453,819	(13,732)	4,484,248	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	290	0	0	290	0	0	0
Unrealized net gain (loss) on investments, net of tax	90,839	0	0	90,839	0	0	0
Change in fair value of cash flow hedges, net of tax	35	0	0	35	0	0	0
Amounts reclassified into earnings on hedging activities, net	724	0	0	724	0	0	0
Net earnings	266,365	0	0	0	266,365	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	348,449	0	0	91,888	256,561	0	0
Balance, end of period at Sep. 30, 2020	4,605,631	10,497	453,819	78,156	4,740,809	(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	(2,400)	0	0	(2,400)	0	0	0	$ 0
Unrealized net gain (loss) on investments, net of tax	(31,645)	0	0	(31,645)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(107)	0	0	(107)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	1,502	0	0	1,502	0	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, net of tax	(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)