Common stock (Parenthetical) (Detail)	Mar. 31, 2016 $ / shares shares
Class A [Member]
Class of Stock [Line Items]
Common Stock, par value per share | $ / shares	$ 5
Common Stock, shares authorized | shares	1,650,000
Class B [Member]
Class of Stock [Line Items]
Common Stock, par value per share | $ / shares	$ 0.0033
Common Stock, shares authorized | shares	3,225,000,000