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Equity-Based Compensation - Weighted Average Assumptions Used for Employee Purchase Rights Granted Under Stock Purchase Plan (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair market value	35.43	34.77	27.95
Option price	33.66	33.03	26.55
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility	21.00%	33.00%	25.00%
Risk free interest rate	0.07%	0.07%	0.14%

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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