Stock-based compensation plans - Summary of Stock Option Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Schedule Of Stock Options [Line Items]
Stock Options Outstanding, Beginning balance	9,839,948
Stock Options Outstanding, Granted	100
Stock Options Outstanding, Exercised	(2,432,316)
Stock Options Outstanding, Cancelled	(1,814)
Stock Options Outstanding, Expired	(372,559)
Stock Options Outstanding, Ending balance	7,033,359
Stock Options Outstanding, Exercisable	7,007,472
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 98.37
Weighted-Average Exercise Price, Granted	$ 79.74
Weighted-Average Exercise Price, Exercised	$ 82.65
Weighted-Average Exercise Price, Cancelled	$ 94.76
Weighted-Average Exercise Price, Expired	$ 107.93
Weighted-Average Exercise Price, Outstanding, Ending balance	$ 103.30
Weighted-Average Exercise Price, Exercisable	$ 103.53
Weighted-Average Life (In Years), Outstanding, Ending balance	3 years 4 months 24 days
Weighted-Average Life (In Years), Exercisable	3 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding, Ending balance	$ 26,212
Aggregate Intrinsic Value, Exercisable	$ 24,761