Stock Options and Other Equity-Based Incentive Instruments - Restricted Stock Unit (RSU) Activity (Detail)	6 Months Ended
	Jun. 30, 2020 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, 2019 (in shares)	68,597
RSUs granted (in shares)	64,036
RSUs added through dividend and performance credits (in shares)	1,344
RSUs released (in shares)	(29,089)
RSUs forfeited/expired (in shares)	(94)
Outstanding at June 30, 2020 (in shares)	104,794
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, 2019 (in shares)	51,312
RSUs granted (in shares)	46,416
RSUs added through dividend and performance credits (in shares)	5,847
RSUs released (in shares)	(20,265)
RSUs forfeited/expired (in shares)	(78)
Outstanding at June 30, 2020 (in shares)	83,232