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Employee Benefits - Employee Share Awards (Detail) - 2006 Equity Incentive Plan
12 Months Ended
Dec. 31, 2015
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Number of Shares, Nonvested - January 1 (shares) | shares 101,181
Number of Shares, Granted (shares) | shares 66,238
Number of Shares, Vested (shares) | shares (62,996)
Number of Shares, Forfeited (shares) | shares (8,368)
Number of Shares, Nonvested - December 31 (shares) | shares 96,055
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]  
Weighted Average Fair Value of the Award, Nonvested - January 1 (dollars per share) | $ / shares $ 152.89
Weighted Average Fair Value of the Award, Granted (dollars per share) | $ / shares 142.50
Weighted Average Fair Value of the Award, Vested (dollars per share) | $ / shares 143.11
Weighted Average Fair Value of the Award, Forfeited (dollars per share) | $ / shares 149.73
Weighted Average Fair Value of the Award, Nonvested - December 31 (dollars per share) | $ / shares $ 150.04