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, 2018	$ 3,408,708	$ 10,497	$ 452,746	$ (4,623)	$ 3,635,561	$ (525,653)	$ (151,997)	$ (7,823)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	260	0	260	0	0	0	0	0
Release of unearned ESOP shares	5,388	0	0	0	0	0	0	5,388
Purchase of ESOP shares	(5,100)	0	0	0	0	0	0	(5,100)
Foreign currency translation	(980)	0	0	(980)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax		0	0	(57,273)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	1,283	0	0	1,283	0	0	0	0
Adjustment for adoption of ASU 2016-01	9,724	0	0	0	9,724	0	0	0
Amounts reclassified into earnings on hedging activities, net	(645)			(645)
Net earnings	291,391	0	0	0	291,391	0	0	0
Common stock dividends	(19,589)	0	0	0	(19,589)	0	0	0
Net activity	224,459	0	260	(57,615)	281,526	0	0	288
Balance, end of period at Sep. 30, 2018	3,633,167	10,497	453,006	(62,238)	3,917,087	(525,653)	(151,997)	(7,535)
Balance, beginning of period at Jun. 30, 2018	3,496,145	10,497	452,862	(45,966)	3,763,339	(525,653)	(151,997)	(6,937)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	144	0	144	0	0	0	0	0
Release of unearned ESOP shares	2,694	0	0	0	0	0	0	2,694
Purchase of ESOP shares	(3,292)	0	0	0	0	0	0	(3,292)
Foreign currency translation	1,113	0	0	1,113	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(17,601)	0	0	(17,601)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	360	0	0	360	0	0	0	0
Amounts reclassified into earnings on hedging activities, net				(144)
Net earnings	163,542	0	0	0	163,542	0	0	0
Common stock dividends	(9,794)	0	0	0	(9,794)	0	0	0
Net activity	137,022	0	144	(16,272)	153,748	0	0	(598)
Balance, end of period at Sep. 30, 2018	3,633,167	10,497	453,006	(62,238)	3,917,087	(525,653)	(151,997)	(7,535)
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		0	0	(1,534)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	(321)			(321)
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
Adjustment for adoption of ASU 2018-02	0	0	0	0	0	0	0	0
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)
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)