ASU 2018 - 12 Transition - (Statement of Operations) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2022
Statements Adjusted [Abstract]
Benefits and losses	$ 167,035	$ 164,079	$ 185,571
Pretax earnings	840,177	1,219,397	1,476,573
Net earnings available to common shareholders	$ 628,707	924,472	1,124,362
Basic earnings per share Common Stock	$ 3.04
Diluted earnings per share Common Stock	$ 3.04
Common Stock [Member]
Statements Adjusted [Abstract]
Net earnings available to common shareholders	$ 0	$ 0	$ 0
Basic earnings per share Common Stock	$ 3.04	$ 5.54	$ 7.08
Diluted earnings per share Common Stock		$ 5.54	$ 7.08
Nonvoting Common Stock [Member]
Statements Adjusted [Abstract]
Net earnings available to common shareholders	$ 0	$ 0	$ 0
Basic earnings per share Common Stock	$ 3.22	$ 4.62	$ 5.58
Diluted earnings per share Common Stock	$ 3.22	$ 4.62	$ 5.58
Before Transition Date [Member]
Statements Adjusted [Abstract]
Benefits and losses		$ 165,553	$ 186,647
Pretax earnings		1,217,923	1,475,497
Net earnings available to common shareholders		$ 922,998	$ 1,123,286
Before Transition Date [Member] | Common Stock [Member]
Statements Adjusted [Abstract]
Basic earnings per share Common Stock		$ 5.54	$ 7.08
Diluted earnings per share Common Stock		5.54	7.08
Before Transition Date [Member] | Nonvoting Common Stock [Member]
Statements Adjusted [Abstract]
Basic earnings per share Common Stock		4.62
Diluted earnings per share Common Stock		$ 4.62	$ 5.58
After Transition Date [Member]
Statements Adjusted [Abstract]
Benefits and losses		$ (1,474)	$ (1,076)
Pretax earnings		1,474	1,076
Net earnings available to common shareholders		$ 1,474	$ 1,076