Shareholders' equity (Tables)

12 Months Ended

Mar. 31, 2020

Stockholders' Equity [Abstract]

Changes in shares of common stock outstanding

	Number of Shares
	Year ended March 31
	2018			2019			2020
Common stock outstanding at beginning of year		3,528,429,451			3,392,937,486			3,310,800,799
Decrease of common stock by cancellation of treasury stock		(179,000,000	)		(150,000,000	)		—
Common stock held in treasury:
Repurchases of common stock		(170,027,391	)		(100,020,867	)		(299,381,781	)
Sales of common stock		201			180			390
Common stock issued to employees		34,115,500			17,894,000			27,168,085
Cancellation of treasury stock		179,000,000			150,000,000			—
Other net change in treasury stock		419,725			(10,000	)		—
Common stock outstanding at end of year		3,392,937,486			3,310,800,799			3,038,587,493