Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jun. 23, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Valuation Adjustment	$ 5,100,000
Auction Rate Securities Purchased as a Result of Settlements with Regulators	140,200,000
Auction Rate Securities Committed To Purchase related to settlements with regulators	3,500,000
Auction Rate Securities Purchased as a result of Legal Settlements and Awards	102,300,000
Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards	3,700,000
Securities owned, at fair value	$ 799,719,000	$ 837,584,000
Loan position held in secondary loan trading portfolio		0
Forward or delayed delivery of the underlying instrument with settlement	180 days
Auction Rate Securities Purchased And Hold	$ 29,400,000
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	32,215,000	28,215,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	25,314,000	37,952,000
Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
ARS Valuation Adjustment For Level Two and Three Assets	4,100,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	$ (21,350,000)	(37,675,000)
Auction rate securities | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitment to purchase ARS period maximum	2020
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		1,096,000
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
Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Unobservable Input Current Yield	4.05%
Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	3.89%
Other Assets | Auction Rate Preferred Securities | Minimum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	2.86%
Fair Value Unobservable Input Current Yield	2.69%
Other Assets | Auction Rate Preferred Securities | Maximum [Member] | Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Input, Discount Rate	3.89%
Fair Value Unobservable Input Current Yield	4.05%
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 133,776,000
Level 2 | Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	25,314,000	$ 16,253,000
6.75% Senior Secured Notes [Member] | Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate			6.75%
Auction Rate Securities Purchase Commitment
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Valuation Adjustment	$ 1,000,000