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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs)
12 Months Ended
Dec. 31, 2019
$ / shares
shares
Service Condition Vesting RSUs  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 66,947
RSUs granted, Number of RSUs 35,273
RSUs added through dividend credits, Number of RSUs 1,314
RSUs released through vesting, Number of RSUs (33,060)
RSUs forfeited/expired, Number of RSUs (1,877)
RSUs outstanding, Number of RSUs, ending balance 68,597
RSUs granted, Weighted Average Fair Value on Date of Grant (in dollars per share) | $ / shares $ 37.41
Market Plus Service Condition Vesting RSUs  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
RSUs outstanding, Number of RSUs, beginning balance 45,536
RSUs granted, Number of RSUs 22,899
Additional market plus service condition RSUs vested, Number of RSUs 7,414
RSUs released through vesting, Number of RSUs (22,237)
RSUs forfeited/expired, Number of RSUs (2,300)
RSUs outstanding, Number of RSUs, ending balance 51,312
RSUs granted, Weighted Average Fair Value on Date of Grant (in dollars per share) | $ / shares $ 31.60