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Cash and Securities Segregated for Regulatory and Other Purposes - Additional Information (Narrative) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Brokers and Dealers [Abstract] |  |
Deposits held in special reserve bank accounts | $ 17.7 us-gaap_CashReserveDepositRequiredAndMade | $ 35.5 us-gaap_CashReserveDepositRequiredAndMade

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- Details
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Namespace Prefix: | us-gaap_
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- Definition Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 305 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478312&loc=d3e61473-112792 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant