ASU 2018 - 12 Transition (Adoption Of New Accounting Pronouncement Adjustments To Select Tables) (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2024	Mar. 31, 2021	Mar. 31, 2020
Statements Adjusted [Abstract]
Assets	$ 18,100,734	$ 17,327,183	$ 19,058,758	$ 14,651,606	$ 14,693,044
Liabilities	11,596,543		11,886,313	9,732,515	$ 9,846,608
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	6,504,191	5,897,161	$ 7,172,445	4,919,091
Cumulative Effect, Period of Adoption, Adjustment [Member]
Statements Adjusted [Abstract]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				$ 72,655
Derecognition of shadow reserves	25,141	26,131
Other adjustments	1,227	1,471
Subtotal	23,914	27,602
After adoption	18,100,734	17,327,183
Cumulative Effect Of Adoption Liabilities [Member]
Statements Adjusted [Abstract]
Derecognition of FIT on shadow reserves	5,280	5,488
Re-measurement due to discount rate	1,626	87,258
Re-measurement due to discount rate FIT	342	18,324
Other adjustments	6,794	8,511
Subtotal	230	82,933
After adoption	11,596,543	11,430,022
Cumulative Effect Of Adoption Comprehensive Income [Member]
Statements Adjusted [Abstract]
Derecognition of shadow reserves	19,861	20,644
Re-measurement due to discount rate	1,626	87,258
Re-measurement due to discount rate FIT	342	18,324
Subtotal	18,577	48,290
After adoption	285,623	1,906
Cumulative Effect Of Adoption Stockholders Equity [Member]
Statements Adjusted [Abstract]
Derecognition of shadow reserves	19,861	20,644
Re-measurement due to discount rate	1,284	68,934
Other adjustments	5,567	7,041
Subtotal	24,144	55,331
After adoption	6,504,191	5,897,161
Scenario Previously Reported [Member]
Statements Adjusted [Abstract]
Assets	18,124,648	17,299,581
Liabilities	11,596,313
Scenario Previously Reported [Member] | Cumulative Effect Of Adoption Liabilities [Member]
Statements Adjusted [Abstract]
Liabilities	11,596,313	11,347,089
Scenario Previously Reported [Member] | Cumulative Effect Of Adoption Comprehensive Income [Member]
Statements Adjusted [Abstract]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	267,046	46,384
Scenario Previously Reported [Member] | Cumulative Effect Of Adoption Stockholders Equity [Member]
Statements Adjusted [Abstract]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 6,528,335	$ 5,952,492