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Stock-based compensation and awards (Details 4)	3 Months Ended		9 Months Ended
Stock-based compensation and awards (Details 4)	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Stock Option Award [Member]
Share based payment award fair value assumptions and methodology
Expected term, (years)	6 years 3 months		6 years 3 months	6 years 3 months
Volatility	37.00%		37.00%	39.00%
Risk-free rate	0.79%		0.79%	2.30%
Risk-free interest rate, Maximum	1.03%		1.26%	2.79%
Dividend yield
2011 ESPP [Member]
Share based payment award fair value assumptions and methodology
Expected term, (years)			6 months
Volatility			30.80%
Risk-free rate			0.15%
Dividend yield

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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