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, 2014	$ 1,527,368	$ 10,497	$ 444,210	$ (53,923)	$ 1,805,453	$ (525,653)	$ (151,997)	$ (1,219)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	5,458	0	5,458	0	0	0	0	0
Release of unearned ESOP Shares	2,767	0	0	0	0	0	0	2,767
Purchase of ESOP share	(7,939)	0	0	0	0	0	0	(7,939)
Foreign currency translation	(19,883)	0	0	(19,883)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	35,190	0	0	35,190	0	0	0	0
Fair market value of cash flow hedges, net of tax	5,086	0	0	5,086	0	0	0	0
Adjustment to postretirement benefit obligation	(835)	0	0	(835)	0	0	0	0
Net earnings	356,741	0	0	0	356,741	0	0	0
Common stock dividends	(19,594)	0	0	0	(19,594)	0	0	0
Net activity	356,991	0	5,458	19,558	337,147	0	0	(5,172)
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	0	1,961	0	0	0	0	0
Release of unearned ESOP Shares	9,507	0	0	0	0	0	0	9,507
Purchase of ESOP share	(9,302)	0	0	0	0	0	0	(9,302)
Foreign currency translation	(4,473)	0	0	(4,473)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(27,066)	0	0	(27,066)	0	0	0	0
Fair market value of cash flow hedges, net of tax	6,027	0	0	6,027	0	0	0	0
Adjustment to postretirement benefit obligation	(648)	0	0	(648)	0	0	0	0
Net earnings	489,001	0	0	0	489,001	0	0	0
Common stock dividends	(97,960)	0	0	0	(97,960)	0	0	0
Net activity	367,047	0	1,961	(26,160)	391,041	0	0	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	0	543	0	0	0	0	0
Release of unearned ESOP Shares	10,360	0	0	0	0	0	0	10,360
Purchase of ESOP share	(11,106)	0	0	0	0	0	0	(11,106)
Foreign currency translation	(5,862)	0	0	(5,862)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	8,984	0	0	8,984	0	0	0	0
Fair market value of cash flow hedges, net of tax	6,149	0	0	6,149	0	0	0	0
Adjustment to postretirement benefit obligation	18	0	0	18	0	0	0	0
Net earnings	398,424	0	0	0	398,424	0	0	0
Common stock dividends	(39,172)	0	0	0	(39,172)	0	0	0
Net activity	368,338	0	543	9,289	359,252	0	0	(746)
Balance at Mar. 31, 2017	$ 2,619,744	$ 10,497	$ 452,172	$ (51,236)	$ 2,892,893	$ (525,653)	$ (151,997)	$ (6,932)