Stock Options and Other Equity-Based Incentive Instruments - Summary of Compensation Costs and Excess Tax Benefits for RSUs (Detail) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Service Condition Vesting RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs for RSUs recognized in income	$ 1,390,000	$ 1,161,237	$ 1,017,000
Excess tax benefit recognized in income Service condition vesting RSUs	372,000	141,000	104,000
Market Plus Service Condition Vesting RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs for RSUs recognized in income	646,000	493,000	370,000
Excess tax benefit recognized in income Service condition vesting RSUs	$ 194,000	$ 146,000	$ 191,000

Amount of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to expense for award under share-based payment arrangement. Excludes expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 740
-Section 35
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=117330312&loc=d3e23163-113944

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Amount of noncash expense for award of restricted stock or unit under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

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