Employee benefit plans and stock-based compensation plans (reflecting the two-for-one stock split (Details)	12 Months Ended
Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Weighted average assumptions to value options and SSRs Granted
Expected volatility of stock	38.40%	37.50%	30.00%
Expected dividend yield	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
Risk-free interest rate	0.50%	1.30%	1.60%
Expected volatility of stock	36.00%	35.00%	30.00%
Maximum [Member]
Weighted average assumptions to value options and SSRs Granted
Expected life of option, in years	5	5	5
Risk-free interest rate	2.40%	2.40%	3.40%
Expected volatility of stock	41.00%	39.00%	37.00%

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)(c)

+ Details

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- Definition

The period of time a share-based award is expected to be outstanding. A share-based award's expected term shall be determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior. An entity is required to aggregate individual awards into relatively homogeneous groups.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)(a)

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

+ Details

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- Definition

The estimated measure of the percentage amount 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)(b)

+ Details

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)(d)

+ Details

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