Derivative Financial Instruments (Summary of Gains and Losses on Derivatives not Designated or Qualifying as Hedges) (Parenthetical) (Detail) - JPY (¥) ¥ in Millions		12 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in income		¥ 481,016	¥ 169,943	¥ (93,321)
Credit-related contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in income	[1]	8,046	(467)	(2,830)
Credit-related contracts | Loans related to credit derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in income		¥ 2,838	¥ (736)	¥ (754)

[1] Amounts include the net gains (losses) of ¥(754) million, ¥(736) million and ¥2,838 million on the credit derivatives economically managing the credit risk of loans during the fiscal years ended March 31, 2018, 2019 and 2020, respectively.