ITG Acquisition - Estimated Fair Values (Details) - USD ($) $ in Thousands	10 Months Ended
	Dec. 31, 2019	Mar. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,148,926		$ 836,583
ITG
Business Acquisition [Line Items]
Cash and equivalents	197,072	$ 197,072
Cash and securities segregated under federal regulations	14,232	14,232
Securities borrowed	13,182	13,182
Receivables from broker dealers and clearing organizations	328,112	328,112
Financial instruments owned, at fair value	523	523
Receivables from customers	122,697	122,697
Property, equipment and capitalized software (net)	46,408	46,408
Intangibles	517,200	479,600
Deferred tax assets	17,605	17,221
Operating lease right-of-use assets	100,285	87,236
Other assets	31,652	31,653
Total Assets	1,388,968	1,337,936
Short-term borrowings	18,651	18,651
Securities loaned	17,663	17,663
Payables to broker dealers and clearing organizations	152,043	152,043
Payables to customers	116,419	116,419
Financial instruments sold, not yet purchased, at fair value	11	11
Accounts payable and accrued expenses and other liabilities	178,893	172,727
Operating lease liabilities	99,693	104,983
Deferred tax liabilities	71,053	65,888
Total Liabilities	654,426	648,385
Total identified assets acquired, net of assumed liabilities	734,542	689,551
Goodwill	312,343	357,334
Total Purchase Price	1,046,885	$ 1,046,885
Measurement Period
Intangibles	37,600
Deferred tax assets	384
Operating lease right-of-use assets	13,049
Other assets	(1)
Total Assets	51,032
Accounts payable and accrued expenses and other liabilities	6,166
Operating lease liabilities	(5,290)
Deferred tax liabilities	5,165
Total Liabilities	6,041
Total identified assets acquired, net of assumed liabilities	44,991
Goodwill	$ (44,991)