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, 2015	$ 1,884,359	$ 10,497	$ 449,668	$ (34,365)	$ 2,142,600	$ (525,653)	$ (151,997)	$ (6,391)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	1,961		1,961
Release of unearned ESOP Shares	9,507							9,507
Purchase of ESOP shares	(9,302)							9,302
Foreign currency translation	(4,473)			(4,473)
Unrealized net gain (loss) on investments, net of tax	(27,066)			(27,066)
Fair market value of cash flow hedges, net of tax	6,027			6,027
Adjustment to postretirement benefit obligation	(648)			(648)
Net earnings	489,001				489,001
Common stock dividends	(97,960)				(97,960)
Net activity	367,047		1,961	(26,160)	391,041			205
Balance at Mar. 31, 2016	2,251,406	10,497	451,629	(60,525)	2,533,641	(525,653)	(151,997)	(6,186)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	543		543
Release of unearned ESOP Shares	10,360							10,360
Purchase of ESOP shares	(11,106)							11,106
Foreign currency translation	(5,862)			(5,862)
Unrealized net gain (loss) on investments, net of tax	8,984			8,984
Fair market value of cash flow hedges, net of tax	6,149			6,149
Adjustment to postretirement benefit obligation	18			18
Net earnings	398,424				398,424
Common stock dividends	(39,172)				(39,172)
Net activity	368,338		543	9,289	359,252			(746)
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				8,740	(8,740)
Increase in market value of released ESOP shares	574		574
Release of unearned ESOP Shares	10,749							10,749
Purchase of ESOP shares	(11,640)							11,640
Foreign currency translation	14,652			14,652
Unrealized net gain (loss) on investments, net of tax	20,104			20,104
Fair market value of cash flow hedges, net of tax	3,082			3,082
Adjustment to postretirement benefit obligation	35			35
Net earnings	790,583				790,583
Common stock dividends	(39,175)				(39,172)
Net activity	788,964		574	46,613	742,668			(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 2018-02	$ (8,700)