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Cash and Securities Segregated Under Federal Regulations and Other Requirements	12 Months Ended
	Dec. 31, 2018
Brokers and Dealers [Abstract]
Cash and Securities Segregated Under Federal Regulations and Other Requirements	Cash and Securities Segregated Under Federal Regulations and Other Requirements As of December 31, 2018 and 2017, $1.2 billion and $0.8 billion, respectively, of U.S. Treasury Bills were segregated in a special reserve bank custody account for the exclusive benefit of our brokerage customers under Rule 15c3-3 of the Exchange Act.