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, 2019	$ 3,692,389	$ 10,497	$ 453,326	$ (66,698)	$ 3,976,962	$ (525,653)	$ (151,997)	$ (4,048)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	435	0	435	0	0	0	0	0
Release of unearned ESOP shares	2,656	0	0	0	0	0	0	2,656
Purchase of ESOP shares	(205)	0	0	0	0	0	0	(205)
Foreign currency translation	3,611	0	0	3,611	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	82,093	0	0	82,093	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(1,534)	0	0	(1,534)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	(321)	0	0	(321)	0	0	0	0
Net earnings	288,748	0	0	0	288,748	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	365,679	0	435	83,849	278,944	0	0	2,451
Balance, end of period at Sep. 30, 2019	4,058,068	10,497	453,761	17,151	4,255,906	(525,653)	(151,997)	(1,597)
Balance, beginning of period at Jun. 30, 2019	3,869,023	10,497	453,535	(23,873)	4,109,384	(525,653)	(151,997)	(2,870)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	226	0	226	0	0	0	0	0
Release of unearned ESOP shares	1,347	0	0	0	0	0	0	1,347
Purchase of ESOP shares	(74)	0	0	0	0	0	0	(74)
Foreign currency translation	629	0	0	629	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	41,305	0	0	41,305	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(633)	0	0	(633)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	(277)	0	0	(277)	0	0	0	0
Net earnings	156,326	0	0	0	156,326	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	189,045	0	226	41,024	146,522	0	0	1,273
Balance, end of period at Sep. 30, 2019	4,058,068	10,497	453,761	17,151	4,255,906	(525,653)	(151,997)	(1,597)
Balance, beginning of period at Mar. 31, 2020	4,220,720	10,497	453,819	34,652	4,399,402	(525,653)	(151,997)	0
Cosolidated statement of change in equity
Adjustment for adoption of ASU 2016-13	(2,880)	0	0	0	(2,880)	0	0	0
Increase in market value of released ESOP shares	0	0	0	0	0	0	0	0
Release of unearned ESOP shares	0	0	0	0	0	0	0	0
Purchase of ESOP shares	0	0	0	0	0	0	0	0
Foreign currency translation	(2,627)	0	0	(2,627)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	45,340	0	0	45,340	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(497)	0	0	(497)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	1,288	0	0	1,288	0	0	0	0
Net earnings	354,091	0	0	0	354,091	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	384,911	0	0	43,504	341,407	0	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)	0
Balance, beginning of period at Jun. 30, 2020	4,257,182	10,497	453,819	(13,732)	4,484,248	(525,653)	(151,997)	0
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	0	0	0	0	0	0	0	0
Release of unearned ESOP shares	0	0	0	0	0	0	0	0
Purchase of ESOP shares	0	0	0	0	0	0	0	0
Foreign currency translation	290	0	0	290	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	90,839	0	0	90,839	0	0	0	0
Change in fair value of cash flow hedges, net of tax	35	0	0	35	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	724	0	0	724	0	0	0	0
Net earnings	266,365	0	0	0	266,365	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	348,449	0	0	91,888	256,561	0	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)	$ 0