Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 48,079
Securities owned, at fair value	$ 911,468	$ 837,584
Forward or delayed delivery of the underlying instrument with settlement	180 days
Auction Rate Securities Committed To Purchase related to settlements with regulators	$ 7,000
Valuation Adjustment For Level Three Asset and Liability	3,811
Fair Value Of Level Three Assets and Liabilities	44,268
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	27,795	28,215
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	40,650
Securities owned, at fair value	37,953	$ 37,952
Valuation adjustment (unrealized loss) for ARS	(2,697)
Total amount of ARS the firm purchased and hold	40,700
Fair Value Of Level Three Assets	$ 37,953
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Committed To Purchase Period	2020
Hedge Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment Redemption Notice Period Minimum	30 days
Investment Redemption Notice Period Maximum	120 days
Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments Lock In Period	10 years
Auction Rate Preferred Securities 01 [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 18,950
Valuation adjustment (unrealized loss) for ARS	(2,697)
Fair Value Of Level Three Assets	$ 16,253
Municipal Auction Rate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	175.00%
U S Treasury Rate	2.49%
Municipal Auction Rate Securities [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 75
Fair Value Unobservable Inputs Duration	2 years
Valuation adjustment (unrealized loss) for ARS	$ 0
Fair Value Unobservable Input Current Yield	5.50%
Fair Value Of Level Three Assets	$ 75
Municipal Auction Rate Securities [Member] \| Weighted Average [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration	2 years
Fair Value Unobservable Input Current Yield	5.50%
Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U S Treasury Rate	2.60%
Auction Rate Preferred Securities \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 21,350
Fair Value Unobservable Inputs Duration	1 year
Valuation adjustment (unrealized loss) for ARS	$ 0
Fair Value Of Level Three Assets	$ 21,350
Auction Rate Preferred Securities \| Weighted Average [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration	1 year
Fair Value Unobservable Input Current Yield	4.07%
Auction Rate Preferred Securities \| Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	110.00%
Auction Rate Preferred Securities \| Minimum [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	2.72%
Auction Rate Preferred Securities \| Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	150.00%
Auction Rate Preferred Securities \| Maximum [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	4.09%
Student Loan Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Spread To U S Treasury Rate	1.20%
U S Treasury Rate	2.48%
Student Loan Auction Rate Securities \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 275
Fair Value Unobservable Inputs Duration	4 years
Valuation adjustment (unrealized loss) for ARS	$ 0
Fair Value Unobservable Input Current Yield	3.95%
Fair Value Of Level Three Assets	$ 275
Student Loan Auction Rate Securities \| Weighted Average [Member] \| Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Inputs Duration	4 years
Fair Value Unobservable Input Current Yield	3.95%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	$ 7,429
Auction Rate Securities Committed To Purchase From Client	7,400
Valuation Adjustment For Level Three Liabilities	1,114
Fair Value Of Level Three Liabilities	6,315
Auction Rate Securities Purchase Commitment \| Auction Rate Preferred Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Level Three Principal Amount	7,429
Valuation Adjustment For Level Three Liabilities	1,114
Fair Value Of Level Three Liabilities	$ 6,315

The total amount of ARS the firm purchased and hold.

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

+ References

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Auction Rate Securities Committed To Purchase related to settlements with regulators

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- Definition

Fair value of the level 3 assets used for the quantitative disclosure.

+ References

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- Definition

Fair Value Of Level Three Assets and Liabilities

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Fair Value Of Level Three Liabilities

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- Definition

Fair value unobservable input current yield.

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- Definition

Fair value unobservable inputs duration.

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- Definition

Investment redemption notice period maximum.

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Investment redemption notice period minimum.

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- Definition

Investments lock in period.

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No definition available.

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- Definition

Amount represents the principal amount of the level 3 assets used for the quantitative disclosure.

+ References

No definition available.

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- Definition

Maximum settlement period in case of delayed delivery of underlying instrument.

+ References

No definition available.

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- Definition

Securities committed to purchase period.

+ References

No definition available.

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- Definition

Spread represents ARS maximum formula which is used to discount for the lack of liquidity as a result of ARS failed auction.

+ References

No definition available.

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- Definition

Specific period of US Treasury rate as of reporting period.

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- Definition

Valuation Adjustment For Level Three Asset and Liability

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- Definition

Valuation adjustment for the level 3 assets used for quantitative disclosure.

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- Definition

Valuation Adjustment For Level Three Liabilities

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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

Fair value of financial instruments held by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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