Stock-based compensation plans (Tables)

9 Months Ended

Sep. 30, 2011

Stock-based compensation plans [Abstract]

Weighted average assumptions to value options and SSRs Granted

	Three Months Ended						Nine Months Ended
	September 30,						September 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		0.3%-2.1	%		0.8%-1.4	%		0.3%-2.2	%		0.8%-2.4	%
Expected volatility of stock		35%-39	%		37%-41	%		35%-39	%		36%-41	%
Expected dividend yield	None			None			None			None