Stock-based compensation plans (Tables)

6 Months Ended

Jun. 30, 2011

Stock-based compensation plans [Abstract]

Weighted average assumptions to value options and SSRs Granted

	Three Months Ended		Six Months Ended
	June 30,		June 30
	2011	2010	2011	2010
Expected life of option	3-5 years	3-5 years	3-5 years	3-5 years
Risk-free interest rate	0.9%-2.0%	1.4%-2.4%	0.9%-2.2%	1.3%-2.4%
Expected volatility of stock	36%-38%	36%-37%	36%-39%	36%-40%
Expected dividend yield	None	None	None	None