Share based compensation plans - Schedule of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding at the beginning of the period (in shares)	2,559,272
Granted	347,250
Exercised	(716,564)
Forfeited	(244,003)
Outstanding at the end of the period (in shares)	1,945,955
Vested and expected to vest	1,903,616
Options exercisable (in shares)	1,448,293
Outstanding at the beginning of the period (in dollars per share)	$ 33.41
Granted	$ 40.46
Exercised	$ 31.50
Forfeited	$ 40.50
Outstanding at the end of the period (in dollars per share)	$ 34.50
Vested and expected to vest	$ 34.42
Options exercisable
Options outstanding, weighted average remaining contractual term	5 years 4 months 21 days
Options vested and expected, weighted average remaining contractual term	5 years 3 months 22 days
Options exercisable, weighted average remaining contractual term	4 years 2 months 12 days