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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, 2019
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 45,536
RSUs granted, Number of RSUs 22,898
RSUs added through dividend and performance credits, Number of RSUs 7,414
RSUs released through vesting, Number of RSUs (22,237)
RSUs outstanding, Number of RSUs, ending balance 53,611
Service Condition Vesting RSUs [Member]  
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 35,272
RSUs added through dividend and performance credits, Number of RSUs 519
RSUs released through vesting, Number of RSUs (26,211)
RSUs outstanding, Number of RSUs, ending balance 76,527