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	116	0	116	0	0	0	0	0
Release of unearned ESOP shares	943	0	0	0	0	0	0	943
Purchase of ESOP shares	(57)	0	0	0	0	0	0	(57)
Foreign currency translation	(2,093)	0	0	(2,093)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(39,672)	0	0	(39,672)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	422	0	0	422	0	0	0	0
Adjustment for adoption of ASU 2016-01	9,724	0	0	0	9,724	0	0	0
Net earnings	127,849	0	0	0	127,849	0	0	0
Common stock dividends	(9,795)	0	0	0	(9,795)	0	0	0
Net activity	87,437	0	116	(41,343)	127,778	0	0	886
Balance, end of period at Jun. 30, 2018	3,496,145	10,497	452,862	(45,966)	3,763,339	(525,653)	(151,997)	(6,937)
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
Adjustment for adoption of ASU 2018-02	0	0	0	0	0	0	0	0
Increase in market value of released ESOP shares	209	0	209	0	0	0	0	0
Release of unearned ESOP shares	1,309	0	0	0	0	0	0	1,309
Purchase of ESOP shares	(131)	0	0	0	0	0	0	(131)
Foreign currency translation	2,982	0	0	2,982	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	40,788	0	0	40,788	0	0	0	0
Change in fair value of cash flow hedges, net of tax		0	0	(945)	0	0	0	0
Net earnings	132,422	0	0	0	132,422	0	0	0
Net activity	176,634	0	209	42,825	132,422	0	0	1,178
Balance, end of period at Jun. 30, 2019	$ 3,869,023	$ 10,497	$ 453,535	$ (23,873)	$ 4,109,384	$ (525,653)	$ (151,997)	$ (2,870)