Common Stock - Schedule of Assumptions Used to Determine Fair Value (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Weighted Average Assumptions For Fair Values Of Performance Based Restricted Stock [Line Items]
Risk-free interest rate	0.38%	0.96%	1.37%
Expected lives (in years)	3 years	3 years	3 years
Expected stock volatility	20.99%	34.61%	35.83%
Dividend yield	4.78%	4.48%	4.90%