Share-based compensation plans (Details 3) (Stock option plans, USD $)	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2009 Y
Stock option plans
Assumptions :
Expected term (in years)	4.58	4.00
Expected volatility minimum (as a percent)	49.60%	45.00%
Expected volatility maximum (as a percent)	49.90%	48.70%
Risk free interest rate minimum (as a percent)	0.90%	1.60%
Risk free interest rate maximum (as a percent)	2.26%	2.57%
Weighted average fair value of options granted during the year (in dollars per share)	$ 14.21	$ 9.29
Changes during the year in company's stock option plan
Outstanding at the beginning of the period (in shares)	2,989,469
Granted (in shares)	386,000
Exercised (in shares)	(587,407)
Forfeited (in shares)	(228,790)
Outstanding at the end of the period (in shares)	2,559,272
Options exercisable (in shares)	2,079,953
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 33.20
Granted (in dollars per share)	$ 33.08
Exercised (in dollars per share)	$ 29.99
Forfeited (in dollars per share)	$ 38.88
Outstanding at the end of the period (in dollars per share)	$ 33.41