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Segregated Cash	6 Months Ended
Segregated Cash	Jun. 30, 2019
Segregated Cash [Abstract]
Segregated Cash	Segregated Cash As of June 30, 2019 and December 31, 2018, cash segregated in compliance with federal regulations and other restricted deposits of $139.1 million and $176.6 million, respectively, consisted of cash deposited in Special Reserve Accounts for the exclusive benefit of customers under SEC Rule 15c3-3 and cash held in accounts designated as Special Reserve Bank Accounts for Proprietary Accounts of Broker-Dealers ("PAB").