Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Revenue	Jun. 30, 2018 USD ($)
Significant Accounting Policies [Line Items]
Operating lease liabilities	$ 89,712,000		$ 89,712,000
Operating lease right-of-use assets	76,722,000		$ 76,722,000
Maximum maturity period for classification of investments as cash equivalents			3 months
Investments other-than-temporary losses	0	$ 0	$ 0	$ 0
Number of revenue streams | Revenue			4
Description of commission revenue			For 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			3 years
Minimum [Member] | Internally Developed Software [Member]
Significant Accounting Policies [Line Items]
Estimated life of intangible assets			3 years
ASU 2016-02
Significant Accounting Policies [Line Items]
Operating lease liabilities	91,200,000		$ 91,200,000
Operating lease right-of-use assets	79,500,000		79,500,000
Deferred rent liability	$ 11,700,000		$ 11,700,000