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, 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	(1,173)	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
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
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	319	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
Balance, beginning of period at Mar. 31, 2020	4,220,720	10,497	453,819	34,652	4,399,402	(525,653)	(151,997)	0
Cosolidated statement of change in equity
Adjustment for adoption of ASU 2018-02	(2,880)	0	0	0	(2,880)	0	0	0
Foreign currency translation	(5,193)	0	0	(5,193)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	54,900	0	0	54,900	0	0	0	0
Change in fair value of cash flow hedges, net of tax	(471)	0	0	(471)	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	2,022	0	0	2,022	0	0	0	0
Net earnings	537,076	0	0	0	537,076	0	0	0
Common stock dividends	(49,019)	0	0	0	(49,019)	0	0	0
Net activity	536,435	0	0	51,258	485,177	0	0	0
Balance, end of period at Dec. 31, 2020	4,757,155	10,497	453,819	85,910	4,884,579	(525,653)	(151,997)	0
Balance, beginning of period at Sep. 30, 2020	4,605,631	10,497	453,819	78,156	4,740,809	(525,653)	(151,997)	0
Cosolidated statement of change in equity
Foreign currency translation	(2,566)	0	0	(2,566)	0	0	0	0
Unrealized net gain (loss) on investments, net of tax	9,560	0	0	9,560	0	0	0	0
Change in fair value of cash flow hedges, net of tax	26	0	0	26	0	0	0	0
Amounts reclassified into earnings on hedging activities, net	734	0	0	734	0	0	0	0
Net earnings	182,985	0	0	0	182,985	0	0	0
Common stock dividends	(39,215)	0	0	0	(39,215)	0	0	0
Net activity	151,524	0	0	7,754	143,770	0	0	0
Balance, end of period at Dec. 31, 2020	$ 4,757,155	$ 10,497	$ 453,819	$ 85,910	$ 4,884,579	$ (525,653)	$ (151,997)	$ 0