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, 2013	$ 1,229,259	$ 10,497	$ 438,168	$ (22,680)	$ 1,482,630	$ (525,653)	$ (151,997)	$ (1,706)
Consolidated statement of change in equity
Increase in market value of released ESOP shares	6,042	0	6,042	0	0	0	0	0
Release of unearned ESOP Shares	694	0	0	0	0	0	0	694
Purchase of ESOP share	(207)	0	0	0	0	0	0	(207)
Foreign currency translation	(9,134)	0	0	(9,134)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	(33,654)	0	0	(33,654)	0	0	0	0
Fair market value of cash flow hedges, net of tax	11,977	0	0	11,977	0	0	0	0
Adjustment to postretirement benefit obligation	(432)	0	0	(432)	0	0	0	0
Net earnings	342,391	0	0	0	342,391	0	0	0
Common stock dividends	(19,568)	0	0	0	(19,568)	0	0	0
Net activity	298,109	0	6,042	(31,243)	322,823	0	0	487
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)