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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, 2018
$ / 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 52,829
RSUs granted, Number of RSUs 16,939
Additional market plus service condition RSUs vested, Number of RSUs 8,506
RSUs released through vesting, Number of RSUs (25,512)
RSUs forfeited/expired, Number of RSUs (970)
RSUs outstanding, Number of RSUs, ending balance 51,792
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 36.40
Service Condition Vesting RSUs [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 68,457
RSUs granted, Number of RSUs 26,939
RSUs added through dividend credits, Number of RSUs 545
RSUs released through vesting, Number of RSUs (25,398)
RSUs forfeited/expired, Number of RSUs (907)
RSUs outstanding, Number of RSUs, ending balance 69,636
RSUs granted, Weighted Average Fair Value on Date of Grant | $ / shares $ 39.31