Acquisitions - Acquisition of Weeden & Co. (Details) - Weeden & Co. - USD ($) $ in Thousands		3 Months Ended
	Aug. 02, 2019	Mar. 31, 2020
Business Combination, Description [Abstract]
Economic value	$ 42,000
Fair value of net assets acquired	24,025
Restricted cash portion of purchase price	10,100
Goodwill recorded	5,800
Customer relationships
Business Combination, Description [Abstract]
Intangible assets acquired	12,000
Internally developed software
Business Combination, Description [Abstract]
Intangible assets acquired	4,700
Net revenue target
Business Combination, Description [Abstract]
Additional cash consideration, maximum amount	31,500
Amount accrued related to earnout		$ 18,500
Non-interest expense recorded related to earnout		$ 17,600
Restricted Stock
Business Combination, Description [Abstract]
Retention award restricted stock	$ 7,300
Requisite service period	4 years