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Commitments and Contingencies - Additional Information (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Eligible investor subject to future buyback principal value	$ 21,900,000
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	88,500,000
Amount of ARS committed to purchase from clients	11,000,000
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	$ 38,000,000

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- Definition

The total amount of ARS the firm purchased and hold.

+ References

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- Definition

The amount of ARS the firm committed to purchase from clients.

+ References

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- Definition

Auction Rate Securities Committed To Purchase related to settlements with regulators

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- Definition

Eligible investor subject to future buyback potential additional losses related to valuation adjustments.

+ References

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- Definition

Eligible investor subject to future buyback principal value.

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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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