Shareholders' equity (Tables)

12 Months Ended

Mar. 31, 2021

Stockholders' Equity [Abstract]

Changes in shares of common stock outstanding

	Number of Shares
	Year ended March 31
	2019			2020			2021
Common stock outstanding at beginning of year		3,392,937,486			3,310,800,799			3,038,587,493
Decrease of common stock by cancellation of treasury stock		(150,000,000	)		—			(260,000,000	)
Common stock held in treasury:
Repurchases of common stock		(100,020,867	)		(299,381,781	)		(20,129	)
Sales of common stock		180			390			353
Common stock issued to employees		17,894,000			27,168,085			24,587,717
Cancellation of treasury stock		150,000,000			—			260,000,000
Other net change in treasury stock		(10,000	)		—			—
Common stock outstanding at end of year		3,310,800,799			3,038,587,493			3,063,155,434