Stock-based compensation plans (Details)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Weighted average assumptions to value options and SSRs Granted
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Weighted average assumptions to value options and SSRs Granted
Expected life of option, in years	3	3	3	3
Risk-free interest rate	0.90%	1.40%	0.90%	1.30%
Expected volatility of stock	36.00%	36.00%	36.00%	36.00%
Maximum [Member]
Weighted average assumptions to value options and SSRs Granted
Expected life of option, in years	5	5	5	5
Risk-free interest rate	2.00%	2.40%	2.20%	2.40%
Expected volatility of stock	38.00%	37.00%	39.00%	40.00%

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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