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]	Cumulative Effect, Period of Adoption, Adjustment [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Retained Earnings [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Retained Earnings [Member]	Nonvoting Common Stock [Member]
Balance at Mar. 31, 2021	$ 4,919,091	$ 10,497	$ 453,819	$ 106,857	$ 5,025,568	$ 525,653	$ 151,997	$ 72,655	$ 64,538	$ 8,117	$ 4,846,436	$ 42,319	$ 5,017,451	$ 0
Consolidated statement of change in equity
Foreign currency translation	2,828	0	0	2,828	0	0	0							0
Unrealized net gain (loss) on investments, net of tax	46,378	0	0	46,378	0	0	0							0
Change in fair value of cash flow hedges, net of tax	457	0	0	457	0	0	0							0
Amounts reclassified into earnings on hedging activities	2,978	0	0	2,978	0	0	0							0
Change in post retirement benefit obligations	1,546	0	0	1,546	0	0	0							0
Net Income (Loss)	1,124,362	0	0	0	1,124,362	0	0							0
Common stock dividends	(29,412)	0	0	0	(29,412)	0	0							0
Series N Non-Voting Common Stock Dividends	0
Net activity	1,050,725	0	0	44,225	1,094,950	0	0							0
Balance at Mar. 31, 2022	5,897,161	10,497	453,819	1,906	6,112,401	525,653	151,997							0
Consolidated statement of change in equity
Common stock dividend	0	0	(176)	0	0	0	0							(176)
Foreign currency translation	782	0	0	782	0	0	0							0
Unrealized net gain (loss) on investments, net of tax	289,477	0	0	289,477	0	0	0							0
Change in fair value of cash flow hedges, net of tax	5,033	0	0	5,033	0	0	0							0
Amounts reclassified into earnings on hedging activities	582	0	0	582	0	0	0							0
Change in post retirement benefit obligations	2,091	0	0	2,091	0	0	0							0
Net Income (Loss)	924,472	0	0	0	628,707	0	0							0
Common stock dividends	(19,608)	0	0	0	(19,608)	0	0							0
Series N Non-Voting Common Stock Dividends	(14,117)	0	0	0	(14,117)	0	0							0
Net activity	607,030	0	176	283,717	890,747	0	0							176
Balance at Mar. 31, 2023	6,504,191	10,497	453,643	285,623	7,003,148	525,653	151,997							176
Consolidated statement of change in equity
Foreign currency translation	2,832	0	0	2,832	0	0	0							0
Unrealized net gain (loss) on investments, net of tax	55,857	0	0	55,857	0	0	0							0
Change in fair value of cash flow hedges, net of tax	6,410	0	0	6,410	0	0	0							0
Amounts reclassified into earnings on hedging activities	4,087	0	0	4,087	0	0	0							0
Change in post retirement benefit obligations	1,395	0	0	1,395	0	0	0							0
Net Income (Loss)	628,707	0	0	0	924,472	0	0							0
Series N Non-Voting Common Stock Dividends	(31,765)	0	0	0	(31,765)	0	0							0
Net activity	668,254	0	8,905	62,407	596,942	0	0							0
Balance at Mar. 31, 2024	$ 7,172,445	$ 10,497	$ 462,548	$ 223,216	$ 7,600,090	$ 525,653	$ 151,997							$ 176