Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail)	6 Months Ended
	Jun. 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)	1,090
RSUs released (in shares)	(42,712)
RSUs forfeited/expired (in shares)	(114)
Outstanding at end of period (in shares)	105,102
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)	0
RSUs released (in shares)	0
RSUs forfeited/expired (in shares)	(75)
Outstanding at end of period (in shares)	113,019