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]
Balance at Mar. 31, 2020	$ 4,287,929	$ 10,497	$ 453,819	$ 34,652	$ 4,466,611	$ (525,653)	$ (151,997)	$ 0
Consolidated statement of change in equity
Adjustment for adoption of ASU 2016-13	(2,880)	0	0	0	(2,880)	0	0	0
Foreign currency translation	(5,694)	0	0	(5,694)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	76,969	0	0	76,969	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(429)	0	0	(429)	0	0	0	0
Amounts reclassified into earnings on hedging activities	2,746	0	0	2,746	0	0	0	0
Change in post retirement benefit obligations	(1,387)	0	0	(1,387)	0	0	0	0
Net earnings	610,856	0	0	0	610,856	0	0	0
Common stock dividends	(49,019)	0	0	0	(49,019)	0	0	0
Series N Non-Voting Common Stock Dividends	0
Net activity	631,162	0	0	72,205	558,957	0	0	0
Balance at Mar. 31, 2021	4,919,091	10,497	453,819	106,857	5,025,568	(525,653)	(151,997)	0
Consolidated statement of change in equity
Foreign currency translation	(2,828)	0	0	(2,828)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(62,626)	0	0	(62,626)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	457	0	0	457	0	0	0	0
Amounts reclassified into earnings on hedging activities	2,978	0	0	2,978	0	0	0	0
Change in post retirement benefit obligations	1,546	0	0	1,546	0	0	0	0
Net earnings	1,123,286	0	0	0	1,123,286	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	1,033,401	0	0	(60,473)	1,093,874	0	0	0
Balance at Mar. 31, 2022	5,952,492	10,497	453,819	46,384	6,119,442	(525,653)	(151,997)	0
Consolidated statement of change in equity
Common stock dividend	0	0	(176)	0	0	0	0	176
Foreign currency translation	(782)	0	0	(782)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(319,190)	0	0	(319,190)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	5,033	0	0	5,033	0	0	0	0
Amounts reclassified into earnings on hedging activities	(582)	0	0	(582)	0	0	0	0
Change in post retirement benefit obligations	2,091	0	0	2,091	0	0	0	0
Net earnings	922,998	0	0	0	922,998	0	0	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0	0
Series N Non-Voting Common Stock Dividends	(14,117)	0	0	0	(14,117)	0	0	0
Net activity	575,843	0	(176)	(313,430)	889,273	0	0	176
Balance at Mar. 31, 2023	$ 6,528,335	$ 10,497	$ 453,643	$ (267,046)	$ 7,008,715	$ (525,653)	$ (151,997)	$ 176