Common stock (Parenthetical) (Detail) (USD $)	Dec. 31, 2014
Class A [Member]
Class of Stock [Line Items]
Common Stock, par value per share	$ 5
Common Stock, shares authorized	1,650,000
Class B [Member]
Class of Stock [Line Items]
Common Stock, par value per share	$ 0.0033
Common Stock, shares authorized	3,225,000,000