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Share-Based Compensation - Summary of Stock Activity under PIP (Detail) (USD $)
1 Months Ended 6 Months Ended
Jan. 31, 2013
Jun. 30, 2013
Directors Stock Compensation Plan [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Number of Shares, Outstanding - December 31, 2012 0 0
Number of Shares, Granted   9,427
Number of Shares, Vested   9,427
Number of Shares, Forfeited   0
Number of Shares, Outstanding - June 30, 2013   0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Weighted Average Fair Value, Outstanding - December 31, 2012 $ 0.00 $ 0.00
Weighted Average Fair Value, Granted   $ 52.49
Weighted Average Fair Value, Vested   $ 52.49
Weighted Average Fair Value, Forfeited   $ 0.00
Weighted Average Fair Value, Outstanding - June 30, 2013   $ 0.00
Performance Incentive Plan [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Number of Shares, Outstanding - December 31, 2012 84,645 84,645
Number of Shares, Granted 23,491 23,491
Number of Shares, Vested   24,332
Number of Shares, Expired   3,043
Number of Shares, Outstanding - June 30, 2013   80,761.000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Weighted Average Fair Value, Outstanding - December 31, 2012 $ 37.86 $ 37.86
Weighted Average Fair Value, Granted   $ 44.85
Weighted Average Fair Value, Vested   $ 33.26
Weighted Average Fair Value, Expired   $ 39.12
Weighted Average Fair Value, Outstanding - June 30, 2013   $ 42.39