Fair Value of Financial Instruments - Information about Significant Unobservable Inputs used in Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 168,626	$ 212,881	$ 168,626	$ 212,881	$ 190,358	$ 171,919	$ 226,339	$ 152,328
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	68,528	99,763	152,878	464,349
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	93,810	121,554	165,540	447,948
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	515
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		870		619
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings	1,386	18,842	9,508	38,765
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings	1,649	(9,639)	(139)	6,006
Minimum [Member]
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			8.7
Minimum [Member] | U.S. government agency securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Market Yields Basis Point	0.11%		0.11%
Maximum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			12
Maximum | U.S. government agency securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Market Yields Basis Point	5.98%		5.98%
Financial instruments and other inventory positions sold, but not yet purchased | Minimum [Member] | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.0382%
Financial instruments and other inventory positions sold, but not yet purchased | Maximum | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.3982%
Financial instruments and other inventory positions sold, but not yet purchased | Weighted Average | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.1594%
Financial instruments and other inventory positions owned
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	98,628	167,302	98,628	167,302	131,791	122,679	172,772	119,083
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,028	99,763	132,378	448,986
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	93,801	111,198	163,172	437,592
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	515
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		251
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings	1,377	12,900	7,140	32,819
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings	718	(6,684)	(397)	4,006
Financial instruments and other inventory positions owned | Convertible securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0		0			1,308
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		1,615
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		251
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings		839
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings		(281)
Financial instruments and other inventory positions owned | Tax-exempt securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	1,306	1,433	1,306	1,433	1,306	1,433	1,433	1,429
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings				(266)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings			(127)	269
Financial instruments and other inventory positions owned | Short-term securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	731	656	731	656	732	656	656	656
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			25
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings			6
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings	(1)		94
Financial instruments and other inventory positions owned | Asset-backed securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	95,274	157,042	95,274	157,042	129,126	119,799	151,396	116,171
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	58,028	99,763	129,764	448,175
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	93,801	97,246	163,047	424,274
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	515
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings	1,377	(276)	9,748	20,562
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings	$ 29	$ 3,405	$ (990)	$ (3,592)
Financial instruments and other inventory positions owned | Minimum [Member] | Discounted cash flow | Level III | Tax-exempt securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Debt service coverage ratio			5.00%
Financial instruments and other inventory positions owned | Minimum [Member] | Discounted cash flow | Level III | Short-term securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)			77.00%
Financial instruments and other inventory positions owned | Minimum [Member] | Discounted cash flow | Level III | Collateralized by Residential mortgages
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate			0.50%
Prepayment rates			2.00%
Loss severity			25.00%
Valuation yields			5.00%
Financial instruments and other inventory positions owned | Minimum [Member] | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.0342%
Financial instruments and other inventory positions owned | Minimum [Member] | Black-Scholes option pricing model | Level III | Warrants in public and private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Liquidity discount rates			30.00%
Financial instruments and other inventory positions owned | Minimum [Member] | Black-Scholes option pricing model | Level III | Warrants in private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Stock volatility factors of comparable companies			23.27%
Financial instruments and other inventory positions owned | Minimum [Member] | Market Comparable | Level III | Equity investment in private company
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple			2
Financial instruments and other inventory positions owned | Maximum | Discounted cash flow | Level III | Tax-exempt securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Debt service coverage ratio			69.00%
Financial instruments and other inventory positions owned | Maximum | Discounted cash flow | Level III | Short-term securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)			80.00%
Financial instruments and other inventory positions owned | Maximum | Discounted cash flow | Level III | Collateralized by Residential mortgages
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate			6.00%
Prepayment rates			22.00%
Loss severity			85.00%
Valuation yields			10.00%
Financial instruments and other inventory positions owned | Maximum | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.1548%
Financial instruments and other inventory positions owned | Maximum | Discounted cash flow | Level III | Equity investment in private company
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple			6.10
Financial instruments and other inventory positions owned | Maximum | Black-Scholes option pricing model | Level III | Warrants in public and private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Liquidity discount rates			40.00%
Financial instruments and other inventory positions owned | Maximum | Black-Scholes option pricing model | Level III | Warrants in private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Stock volatility factors of comparable companies			93.60%
Financial instruments and other inventory positions owned | Weighted Average | Discounted cash flow | Level III | Tax-exempt securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Debt service coverage ratio			22.20%
Financial instruments and other inventory positions owned | Weighted Average | Discounted cash flow | Level III | Short-term securities
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)			79.60%
Financial instruments and other inventory positions owned | Weighted Average | Discounted cash flow | Level III | Collateralized by Residential mortgages
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate			3.36%
Prepayment rates			4.28%
Loss severity			66.03%
Valuation yields			5.06%
Financial instruments and other inventory positions owned | Weighted Average | Discounted cash flow | Level III | Interest rate derivative contract
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Unamortized premium over the MMD curve			0.1143%
Financial instruments and other inventory positions owned | Weighted Average | Discounted cash flow | Level III | Equity investment in private company
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple			3.6
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple			9.4
Financial instruments and other inventory positions owned | Weighted Average | Black-Scholes option pricing model | Level III | Warrants in public and private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Liquidity discount rates			35.50%
Financial instruments and other inventory positions owned | Weighted Average | Black-Scholes option pricing model | Level III | Warrants in private companies
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Stock volatility factors of comparable companies			47.90%