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, 2017	$ 2,619,744	$ 10,497	$ 452,172	$ (51,236)	$ 2,892,893	$ (525,653)	$ (151,997)	$ (6,932)
Consolidated statement of change in equity
Adjustment for adoption of ASU 2018-02	0	0	0	8,740	(8,740)	0	0	0
Increase in market value of released ESOP shares	574	0	574	0	0	0	0	0
Release of unearned ESOP Shares	10,749	0	0	0	0	0	0	10,749
Purchase of ESOP shares	(11,640)	0	0	0	0	0	0	(11,640)
Foreign currency translation	14,652	0	0	14,652	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	20,104	0	0	20,104	0	0	0	0
Fair market value of cash flow hedges, net of tax	3,082	0	0	3,082	0	0	0	0
Adjustment to postretirement benefit obligation	35	0	0	35	0	0	0	0
Net earnings	790,583	0	0	0	790,583	0	0	0
Common stock dividends	(39,175)	0	0	0	(39,175)	0	0	0
Net activity	788,964	0	574	46,613	742,668	0	0	(891)
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
Adjustment for 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		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		0	0	(50,044)	0	0	0	0
Fair market value of cash flow hedges, net of tax		0	0	477	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
Adjustment for adoption of ASU 2018-02	(8,700)
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		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
Fair market value of cash flow hedges, net of tax		0	0		0	0	0	0
Adjustment to postretirement benefit obligation	333	0	0	333	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