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, 2018	$ 3,408,708	$ 10,497	$ 452,746	$ (4,623)	$ 3,635,561	$ (525,653)	$ (151,997)	$ (7,823)
Consolidated statement of change in equity
Unrealized net loss on investments, less adoption of ASU 2016-01	0	0	0	(9,724)	9,724	0	0	0
Increase in market value of released ESOP shares	580	0	580	0	0	0	0	0
Release of unearned ESOP Shares	9,392	0	0	0	0	0	0	9,392
Purchase of ESOP shares	(5,617)	0	0	0	0	0	0	(5,617)
Foreign currency translation	(1,759)	0	0	(1,759)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(50,044)	0	0	(50,044)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	451	0	0	451	0	0	0	0
Amounts reclassified into earnings on hedging activities	26	0	0	26	0	0	0	0
Adjustment to postretirement benefit obligation	(1,025)	0	0	(1,025)	0	0	0	0
Net earnings	370,857	0	0	0	370,857	0	0	0
Common stock dividends	(39,180)	0	0	0	(39,180)	0	0	0
Net activity	283,681	0	580	(62,075)	341,401	0	0	3,775
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
Purchase of ESOP shares	0	0	0	0	0	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