Weighted-Average Assumptions used to Estimate Fair Value of Non-Vested Shares Granted (Detail) (Employee Non vested stock options)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Non vested stock options
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Weighted Average Fair Values [Line Items]
Risk-free interest rate	0.37%	1.04%	1.25%
Expected dividend yield	2.24%	2.34%	2.24%