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Benefit Plans - Summary of Fair Values for Stock-based Awards Granted Using Black-Scholes Option Pricing and Monte Carlo Approach Model (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of grants $ 5.07 $ 5.63 $ 5.30
Risk-free interest rate (range), minimum 0.50% 1.40% 1.40%
Risk-free interest rate (range), maximum 1.10% 3.00% 2.60%
Dividend yield (range), minimum 3.60% 3.40% 4.20%
Dividend yield (range), maximum 4.70% 4.90% 5.60%
Expected life   7 years  
Expected volatility (range), minimum 50.80% 52.10% 87.00%
Expected volatility (range), maximum 66.80% 69.00% 97.80%
VSEP awards [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 1.90%    
Dividend yield 6.20%    
Expected life 3 years 4 months 24 days    
Expected volatility 88.00%    
Minimum [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 4 years   4 years
Maximum [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 5 years   5 years