Calculation and Assumptions Utilized in Calculation of Fair Values of Options Granted (Detail) (Performance Stock Unit, USD $)	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012
Performance Stock Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares base award	234,117	282,040
Grant date per share fair value (in dollars per share)	$ 27.60	$ 23.79
Risk-free interest rate	0.40%	0.40%
Expected life in years	3 years	3 years
Expected volatility (over expected life)	29.10%	35.00%
Expected dividend yield (over expected life)	4.20%	4.80%

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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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