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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs) [Member]
6 Months Ended
Jun. 30, 2016
$ / shares
shares
Market Plus Service Condition Vesting RSUs [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 32,097
RSUs granted, Number of RSUs 18,753
RSUs outstanding, Number of RSUs, ending balance 50,850
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 24.39
Service Condition Vesting RSUs [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 46,286
RSUs granted, Number of RSUs 36,542
RSUs added through dividend credits, Number of RSUs 641
RSUs released, Number of RSUs (16,948)
RSUs forfeited/expired, Number of RSUs (4)
RSUs outstanding, Number of RSUs, ending balance 66,517
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 26.60