Segregated Cash	3 Months Ended
Mar. 31, 2021
Segregated Cash [Abstract]
Segregated Cash	Segregated CashAs of March 31, 2021 and December 31, 2020, cash segregated in compliance with federal regulations and other restricted deposits of $168.9 million and $185.1 million, respectively, consisted of cash deposited in Special Reserve Bank Accounts for the exclusive benefit of customers under SEC Rule 15c3-3 and cash deposited in Special Reserve Bank Accounts for the exclusive benefit of Proprietary Accounts of Broker-Dealers ("PAB") under SEC Rule 15c3-3

The entire disclosure for assets that are restricted in their use, generally by contractual agreements or regulatory requirements. This would include, but not limited to, a description of the restricted assets and the terms of the restriction.

No definition available.

Name:	us-gaap_RestrictedAssetsDisclosureTextBlock
Namespace Prefix:	us-gaap_
Data Type:	nonnum:textBlockItemType
Balance Type:	na
Period Type:	duration