Accounting Pronouncements (Effects of Adoption of ASU 2018-12 Statement Of Operations) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statements Adjusted [Abstract]
Benefits and losses	$ 42,553	$ 39,512	$ 87,897	$ 79,269
Amortization of deferred policy acquisition costs	6,826	6,972	14,871	14,644
Pretax earnings	358,048	461,490	696,743	906,889
Net earnings available to common shareholders	273,508	349,866	530,346	688,211
Common Stock [Member]
Statements Adjusted [Abstract]
Net earnings available to common shareholders	$ 0	$ 0	$ 0	$ 0
Basic and diluted earnings per share Common Stock	$ 1.36	$ 2.23	$ 2.63	$ 4.41
Nonvoting Common Stock [Member]
Statements Adjusted [Abstract]
Net earnings available to common shareholders	$ 0	$ 0	$ 0	$ 0
Basic and diluted earnings per share Common Stock	$ 1.40	$ 1.73	$ 2.71	$ 3.41
Operating Segments [Member] | Life Insurance [Member]
Statements Adjusted [Abstract]
Benefits and losses	$ 38,324	$ 33,437	$ 79,210	$ 68,815
Amortization of deferred policy acquisition costs	6,826	6,972	14,871	14,644
Pretax earnings	5,488	(417)	6,724	5,379
Net earnings available to common shareholders	4,624	$ (613)	5,344	$ 4,800
Previously Reported
Statements Adjusted [Abstract]
Benefits and losses	37,363		81,463
Pretax earnings	463,639		904,695
Net earnings available to common shareholders	$ 352,015		$ 686,017
Basic and diluted earnings per share Common Stock	$ 2.25		$ 4.4
Previously Reported | Nonvoting Common Stock [Member]
Statements Adjusted [Abstract]
Basic and diluted earnings per share Common Stock	$ 1.75		$ 3.4
Effect of adoption
Statements Adjusted [Abstract]
Benefits and losses	$ 2,149		$ (2,194)
Pretax earnings	(2,149)		2,194
Net earnings available to common shareholders	$ (2,149)		$ 2,194
Basic and diluted earnings per share Common Stock	$ (0.02)		$ 0.01
Effect of adoption | Nonvoting Common Stock [Member]
Statements Adjusted [Abstract]
Basic and diluted earnings per share Common Stock	$ (0.02)		$ 0.01