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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail)
9 Months Ended
Sep. 30, 2021
shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
RSUs released (in shares) 0
Restricted Stock Units (RSUs) | Service Condition Vesting RSUs  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Outstanding at beginning of period (in shares) 99,809
RSUs granted (in shares) 47,029
RSUs added through dividend and performance credits (in shares) 1,687
RSUs released (in shares) (45,401)
RSUs forfeited/expired (in shares) (190)
Outstanding at end of period (in shares) 102,934
Restricted Stock Units (RSUs) | Market Plus Service Condition Vesting RSUs  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Outstanding at beginning of period (in shares) 81,615
RSUs granted (in shares) 31,479
RSUs added through dividend and performance credits (in shares) 6,067
RSUs released (in shares) (19,272)
RSUs forfeited/expired (in shares) (126)
Outstanding at end of period (in shares) 99,763