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 at Mar. 31, 2019	$ 3,692,389	$ 10,497	$ 453,326	$ (66,698)	$ 3,976,962	$ (525,653)	$ (151,997)	$ (4,048)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	493	0	493	0	0	0	0	0
Release of unearned ESOP Shares	4,253	0	0	0	0	0	0	4,253
Purchase of ESOP shares	(205)	0	0	0	0	0	0	(205)
Foreign currency translation	9,377	0	0	9,377	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	97,943	0	0	97,943	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(6,301)	0	0	(6,301)	0	0	0	0
Amounts reclassified into earnings on hedging activities	(2)	0	0	(2)	0	0	0	0
Adjustment to postretirement benefit obligation	333	0	0	333	0	0	0	0
Net earnings	442,048	0	0	0	442,048	0	0	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0	0
Net activity	528,331	0	493	101,350	422,440	0	0	4,048
Balance at Mar. 31, 2020	4,220,720	10,497	453,819	34,652	4,399,402	(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
Adjustment to postretirement benefit obligation	(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
Net activity	631,162	0	0	72,205	558,957	0	0	0
Balance at Mar. 31, 2021	4,851,882	10,497	453,819	106,857	4,958,359	(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
Adjustment to postretirement benefit obligation	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
Net activity	1,033,401	0	0	(60,473)	1,093,874	0	0	0
Balance at Mar. 31, 2022	$ 5,885,283	$ 10,497	$ 453,819	$ 46,384	$ 6,052,233	$ (525,653)	$ (151,997)	$ 0