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]
Balance, beginning of period at Mar. 31, 2020	$ 4,220,720	$ 10,497	$ 453,819	$ 34,652	$ 4,399,402	$ (525,653)	$ (151,997)
Cosolidated statement of change in equity
Adjustment for adoption of ASU 2016-13	(2,880)	0	0	0	(2,880)	0	0
Foreign currency translation	(2,917)	0	0	(2,917)	0	0	0
Unrealized net gain (loss) on investments, net of tax	(45,499)	0	0	(45,499)	0	0	0
Change in fair value of cash flow hedges, net of tax	(532)	0	0	(532)	0	0	0
Amounts reclassified into earnings on hedging activities	564	0	0	564	0	0	0
Net earnings	87,726	0	0	0	87,726	0	0
Net activity	36,462	0	0	(48,384)	84,846	0	0
Balance, end of period at Jun. 30, 2020	4,257,182	10,497	453,819	(13,732)	4,484,248	(525,653)	(151,997)
Balance, beginning of period at Mar. 31, 2021	4,851,882	10,497	453,819	106,857	4,958,359	(525,653)	(151,997)
Cosolidated statement of change in equity
Foreign currency translation	(3,392)	0	0	(3,392)	0	0	0
Unrealized net gain (loss) on investments, net of tax	(73,027)	0	0	(73,027)	0	0	0
Change in fair value of cash flow hedges, net of tax	(51)	0	0	(51)	0	0	0
Amounts reclassified into earnings on hedging activities	745	0	0	745	0	0	0
Net earnings	345,175	0	0	0	345,175	0	0
Common stock dividends	(9,804)	0	0	0	(9,804)	0	0
Net activity	259,646	0	0	(75,725)	335,371	0	0
Balance, end of period at Jun. 30, 2021	$ 5,111,528	$ 10,497	$ 453,819	$ 31,132	$ 5,293,730	$ (525,653)	$ (151,997)