Condensed 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, beginning of period at Mar. 31, 2018	$ 3,408,708	$ 10,497	$ 452,746	$ (4,623)	$ 3,635,561	$ (525,653)	$ (151,997)	$ (7,823)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	370	0	370	0	0	0	0	0
Release of unearned ESOP shares	8,083	0	0	0	0	0	0	8,083
Purchase of ESOP shares	(5,403)	0	0	0	0	0	0	(5,403)
Foreign currency translation	(5,081)	0	0	(5,081)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax		0	0	(55,830)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	528	0	0	528	0	0	0	0
Adjustment for adoption of ASU 2016-01	0	0	0	(9,724)	9,724	0	0	0
Amounts reclassified into earnings on hedging activities, net	23	0	0	23		0	0	0
Net earnings	370,026	0	0	0	370,026	0	0	0
Common stock dividends	(29,384)	0	0	0	(29,384)	0	0	0
Net activity	283,332	0	370	(70,084)	350,366	0	0	2,680
Balance, end of period at Dec. 31, 2018	3,692,040	10,497	453,116	(74,707)	3,985,927	(525,653)	(151,997)	(5,143)
Balance, beginning of period at Sep. 30, 2018	3,633,167	10,497	453,006	(62,238)	3,917,087	(525,653)	(151,997)	(7,535)
Cosolidated statement of change in equity
Increase in market value of released ESOP shares	110	0	110	0	0	0	0	0
Release of unearned ESOP shares	2,695	0	0	0	0	0	0	2,695
Purchase of ESOP shares	(303)	0	0	0	0	0	0	(303)
Foreign currency translation	(4,101)	0	0	(4,101)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax		0	0	(8,281)	0	0	0	0
Change in fair value of cash flow hedges, net of tax	57	0	0	57	0	0	0	0
Amounts reclassified into earnings on hedging activities, net		0	0	(144)	0	0	0	0
Net earnings	78,635	0	0	0	78,635	0	0	0
Common stock dividends	(9,795)	0	0	0	(9,795)	0	0	0
Net activity	58,873	0	110	(12,469)	68,840	0	0	2,392
Balance, end of period at Dec. 31, 2018	3,692,040	10,497	453,116	(74,707)	3,985,927	(525,653)	(151,997)	(5,143)
Balance, beginning of period at Mar. 31, 2019	3,692,389	10,497	453,326	(66,698)	3,976,962	(525,653)	(151,997)	(4,048)
Cosolidated statement of change in equity
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	(205)	0	0	0	0	0	0	(205)
Foreign currency translation	2,919	0	0	2,919	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	104,471	0	0	104,471	0	0	0	0
Change in fair value of cash flow hedges, net of tax		0	0	(1,173)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	(2)	0	0	(2)	0	0	0	0
Net earnings	319,680	0	0	0	319,680	0	0	0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0	0
Net activity	410,828	0	493	106,215	300,072	0	0	4,048
Balance, end of period at Dec. 31, 2019	4,103,217	10,497	453,819	39,517	4,277,034	(525,653)	(151,997)	0
Balance, beginning of period at Sep. 30, 2019	4,058,068	10,497	453,761	17,151	4,255,906	(525,653)	(151,997)	(1,597)
Cosolidated statement of change in equity
Adjustment for adoption of ASU 2018-02	0	0	0	0	0	0	0	0
Increase in market value of released ESOP shares	58	0	58	0	0	0	0	0
Release of unearned ESOP shares	1,597	0	0	0	0	0	0	1,597
Purchase of ESOP shares	0	0	0	0	0	0	0	0
Foreign currency translation	(692)	0	0	(692)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	22,378	0	0	22,378	0	0	0	0
Change in fair value of cash flow hedges, net of tax	361	0	0	361	0	0	0	0
Amounts reclassified into earnings on hedging activities, net		0	0	319	0	0	0	0
Net earnings	30,932	0	0	0	30,932	0	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0	0
Net activity	45,149	0	58	22,366	21,128	0	0	1,597
Balance, end of period at Dec. 31, 2019	$ 4,103,217	$ 10,497	$ 453,819	$ 39,517	$ 4,277,034	$ (525,653)	$ (151,997)	$ 0