Shareholders' equity (Tables)

12 Months Ended

Mar. 31, 2019

Stockholders' Equity [Abstract]

Changes in shares of common stock outstanding

	Number of Shares
	Year ended March 31
	2017			2018			2019
Common stock outstanding at beginning of year		3,608,391,999			3,528,429,451			3,392,937,486
Decrease of common stock by cancellation of treasury stock		—			(179,000,000	)		(150,000,000	)
Common stock held in treasury:
Repurchases of common stock		(121,010,524	)		(170,027,391	)		(100,020,867	)
Sales of common stock		468			201			180
Common stock issued to employees		40,677,400			34,115,500			17,894,000
Cancellation of treasury stock		—			179,000,000			150,000,000
Other net change in treasury stock		370,108			419,725			(10,000	)
Common stock outstanding at end of year		3,528,429,451			3,392,937,486			3,310,800,799