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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares
3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2015
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   15,366
RSUs granted, Number of RSUs   18,348
RSUs forfeited/expired, Number of RSUs   (1,617)
RSUs outstanding, Number of RSUs, ending balance 15,366 32,097
RSUs granted, Weighted Average Fair Value on Date of Grant   $ 21.01
Service Condition Vesting RSUs [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
RSUs outstanding, Number of RSUs, beginning balance   30,920
RSUs granted, Number of RSUs 13,749 30,348
RSUs added through dividend credits, Number of RSUs   962
RSUs released, Number of RSUs   (12,064)
RSUs forfeited/expired, Number of RSUs   (3,880)
RSUs outstanding, Number of RSUs, ending balance 30,920 46,286
RSUs granted, Weighted Average Fair Value on Date of Grant   $ 23.45