Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Revenue	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Maximum maturity period for classification of investments as cash equivalents	3 months
Investments other-than-temporary losses	$ 0	$ 0	$ 0
Allowance for credit losses	163,000	57,000	100,000
Provision for bad debts	500,000	300,000	200,000
Write-offs and other charges against the allowance for credit losses	$ 100,000	$ 100,000	$ 200,000
Number of revenue streams | Revenue	4
Description of commission revenue	For Open Trading trades that the Company executes between and among institutional investor and broker-dealer clients on a matched principal basis by serving as counterparty to both the buyer and the seller, the Company earns its commission through the difference in price between the two trades.
Settlement days of bond transaction	within one to two trading days
Maximum [Member]
Significant Accounting Policies [Line Items]
Contractual maturities accounts receivable	1 year
Estimated useful life of fixed assets	7 years
Maximum [Member] | Business Combinations [Member]
Significant Accounting Policies [Line Items]
Estimated life of intangible assets	15 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of fixed assets	3 years
Minimum [Member] | Business Combinations [Member]
Significant Accounting Policies [Line Items]
Estimated life of intangible assets	1 year
Minimum [Member] | Internally Developed Software [Member]
Significant Accounting Policies [Line Items]
Estimated life of intangible assets	3 years
ASU 2018-15 [Member]
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jul. 01, 2019
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Retrospective Application Impracticable [true false]	false
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU 2017-04 [Member]
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Retrospective Application Impracticable [true false]	false
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU 2016-13 [Member]
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Retrospective Application Impracticable [true false]	true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true