Fair Values of all Options Granted Estimated Using Binomial Option-Pricing Model with Weighted-average Assumptions (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	2.80%	2.17%
Weighted average expected volatility	53.87%	50.90%
Weighted average risk-free interest rate	0.83%	1.14%
Weighted average expected lives (in years)	5 years 4 months 2 days	5 years 4 months 6 days
Weighted average grant-date fair value		$ 7.85