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Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail)
3 Months Ended
Mar. 31, 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 December 31, 2020 (in shares) 99,809
RSUs granted (in shares) 0
RSUs added through dividend and performance credits (in shares) 504
RSUs released (in shares) (201)
RSUs forfeited/expired (in shares) (55)
Outstanding at March 31, 2021 (in shares) 100,057
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 December 31, 2020 (in shares) 81,615
RSUs granted (in shares) 0
RSUs added through dividend and performance credits (in shares) 0
RSUs released (in shares) 0
RSUs forfeited/expired (in shares) 0
Outstanding at March 31, 2021 (in shares) 81,615