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Stock-Based Compensation - Summary of Nonvested Common Stock Units and Share Awards (Detail) - Common Stock Awards [Member] - Non-Employee Director Fee Plan [Member] - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Nonvested Shares, beginning balance 12    
Granted, Shares 32 36 37
Vested, Shares (33)    
Forfeited, Shares 0    
Nonvested Shares, ending balance 11 12  
Nonvested, Weighted Average Grant-Date Fair Value, beginning balance $ 20.24    
Granted, Weighted Average Grant-Date Fair Value 24.70 $ 20.15 $ 16.01
Vested, Weighted Average Grant-Date Fair Value 23.43    
Forfeited or expired, Weighted Average Grant-Date Fair Value 0    
Nonvested, Weighted Average Grant-Date Fair Value, ending balance $ 23.74 $ 20.24