Consolidated Statements of Changes in Stockholders' Equity (USD $) In Thousands, unless otherwise specified	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, 2012	$ 1,035,820	$ 10,497	$ 433,743	$ (45,436)	$ 1,317,064	$ (525,653)	$ (151,997)	$ (2,398)
Consolidated statement of change in equity
Adjustment to initially apply ASU 2010-26	(1,721)	0	0	0	(1,721)	0	0	0
Increase in market value of released ESOP shares	4,425	0	4,425	0	0	0	0	0
Release of unearned ESOP Shares	692	0	0	0	0	0	0	692
Foreign currency translation	(1,271)	0	0	(1,271)	0	0	0	0
Unrealized gain on investments, net of tax	18,779	0	0	18,779	0	0	0	0
Fair market value of cash flow hedges, net of tax	5,831	0	0	5,831	0	0	0	0
Adjustment to postretirement benefit obligation	(583)	0	0	(583)	0	0	0	0
Net earnings	264,708	0	0	0	264,708	0	0	0
Common stock dividends	(97,421)	0	0	0	(97,421)	0	0	0
Net activity	193,439	0	4,425	22,756	165,566	0	0	692
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	487	0	0	0	0	0	0	487
Foreign currency translation	(9,134)	0	0	(9,134)	0	0	0	0
Unrealized gain 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	265	0	0	0	0	0	0	265
Purchase of ESOP shares	(5,437)	0	0	0	0	0	0	(5,437)
Foreign currency translation	(19,883)	0	0	(19,883)	0	0	0
Unrealized gain 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)