Accounting Pronouncements (Consolidated Balance Sheet Before After Transition Date (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Mar. 31, 2021
Statements Adjusted [Abstract]
Deferred policy acquisition costs, net	$ 121,365	$ 128,463
Assets	19,021,450	18,100,734	$ 18,143,343
Policy benefits and losses, claims and loss expenses payable	865,397	880,202
Deferred income taxes, net	1,444,120	1,329,489
Total liabilities	11,991,072	11,596,543
Accumulated other comprehensive loss	(275,664)	(285,623)
Retained earnings	7,519,376	7,003,148
Stockholders' Equity Attributable to Parent	7,030,378	6,504,191
Liabilities and Equity	$ 19,021,450	$ 18,100,734
Before Transition Date [Member]
Statements Adjusted [Abstract]
Deferred policy acquisition costs, net				$ 89,749
Assets				14,651,606
Policy benefits and losses, claims and loss expenses payable				909,701
Deferred income taxes, net				1,199,280
Total liabilities				9,732,515
Accumulated other comprehensive loss				106,857
Retained earnings				5,025,568
Stockholders' Equity Attributable to Parent				4,919,091
Liabilities and Equity				14,651,606
After Transition Date [Member]
Statements Adjusted [Abstract]
Deferred policy acquisition costs, net				131,187
Assets				14,693,044
Policy benefits and losses, claims and loss expenses payable				1,040,951
Deferred income taxes, net				1,182,123
Total liabilities				9,846,608
Accumulated other comprehensive loss				42,319
Retained earnings				5,017,451
Stockholders' Equity Attributable to Parent				4,846,436
Liabilities and Equity				$ 14,693,044