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Commitments and Contingencies - Additional Information (Narrative) (Details)	6 Months Ended
	Jun. 30, 2018 USD ($)
Loss Contingencies [Line Items]
Eligible investor subject to future buyback principal value	$ 21,200,000
Minimum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings	0
Eligible investor subject to future buyback potential additional losses related to valuation adjustments	0
Maximum [Member]
Loss Contingencies [Line Items]
Maximum estimated range of aggregate loss for legal proceedings	30,000,000
Auction rate securities
Loss Contingencies [Line Items]
Auction Rate Securities Committed To Purchase related to settlements with regulators	5,000,000
Total amount of ARS the firm purchased and hold	$ 92,300,000

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The total amount of ARS the firm purchased and hold.

+ References

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- Definition

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- Definition

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+ References

No definition available.

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- Definition

Eligible investor subject to future buyback principal value.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6395354&loc=d3e12631-108344

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