Share Based Compensation Weighted average assumptions by grant year (Details) (USD $)	12 Months Ended
Jan. 31, 2012 Y	Jan. 31, 2011 Y	Jan. 31, 2010 Y
Share-based Compensation [Abstract]
Risk-free interest rate	0.67%	1.46%	2.03%
Expected dividend yield	1.20%	1.49%	1.73%
Expected volatility factor	51.44%	49.33%	49.69%
Expected option term (in years)	4.00	4.50	4.50
Weighted average grant date fair value	$ 22.11	$ 15.70	$ 11.28

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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