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Assumptions Used to Determine Weighted Average Grant Date Fair Value (Detail) (Restricted Shares With Common Stock Price Thresholds)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Restricted Shares With Common Stock Price Thresholds
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	40.70%	45.80%	44.00%
Risk-free interest rates	0.80%	0.90%	2.20%
Derived service periods (in years)	0.1	0.2	0.2

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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