Derivative Financial Instruments and Hedging (Effect of Derivative Instruments on Consolidated Statements of Income, Pre-tax, Derivatives not designated as hedging instruments) (Detail) - Nondesignated - JPY (¥) ¥ in Millions	6 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Interest rate swap agreements | Life insurance premiums and related investment income
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	¥ 0	¥ 0
Interest rate swap agreements | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	2
Interest rate swap agreements | Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	8	(8)
Futures | Life insurance premiums and related investment income
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	(141)	(1,557)
Futures | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	0
Futures | Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	1,259	(685)
Foreign exchange contracts | Life insurance premiums and related investment income
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	6,698	30,067
Foreign exchange contracts | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	(10,630)	1,513
Foreign exchange contracts | Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	44,741	5,077
Credit derivatives held/written | Life insurance premiums and related investment income
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	0
Credit derivatives held/written | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	0
Credit derivatives held/written | Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	8
Options held/written and other | Life insurance premiums and related investment income
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	0
Options held/written and other | Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	0	0
Options held/written and other | Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recognized in income on derivative	¥ (522)	¥ (878)