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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, 2020
$ / shares
shares
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, beginning balance (in shares) 68,597
RSUs granted (in shares) 64,036
Additional market plus service condition RSUs vested (in shares) 0
RSUs added through dividend credits (in shares) 2,937
RSUs released through vesting (in shares) (34,388)
RSUs forfeited/expired (in shares) (1,373)
Outstanding, ending balance (in shares) 99,809
RSUs granted, weighted average fair value on date of grant (in USD per share) | $ / shares $ 31.26
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, beginning balance (in shares) 51,312
RSUs granted (in shares) 46,416
Additional market plus service condition RSUs vested (in shares) 5,847
RSUs added through dividend credits (in shares) 0
RSUs released through vesting (in shares) (20,265)
RSUs forfeited/expired (in shares) (1,695)
Outstanding, ending balance (in shares) 81,615
RSUs granted, weighted average fair value on date of grant (in USD per share) | $ / shares $ 23.30