Fair Value of Financial Instruments - Changes in Fair Value Associated with Level III Financial Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfers between fair value levels			$ 0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	190,358,000	226,339,000	171,919,000	152,328,000
Purchases	68,528,000	99,763,000	152,878,000	464,349,000
Sales	(93,810,000)	(121,554,000)	(165,540,000)	(447,948,000)
Transfers in	515,000
Transfers out		(870,000)		(619,000)
Realized gains/ (losses)	1,386,000	18,842,000	9,508,000	38,765,000
Unrealized gains/ (losses)	1,649,000	(9,639,000)	(139,000)	6,006,000
Ending balance	168,626,000	212,881,000	168,626,000	212,881,000
Financial instruments and other inventory positions sold, but not yet purchased
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	10,846,000	254,000	6,643,000	5,218,000
Purchases	(8,439,000)	(223,000)	(16,751,000)	(5,873,000)
Sales				745,000
Transfers in
Transfers out
Realized gains/(losses)	8,439,000	223,000	16,751,000	5,115,000
Unrealized gains/ (losses)	(679,000)	6,327,000	3,524,000	1,376,000
Ending balance	10,167,000	6,581,000	10,167,000	6,581,000
Financial instruments and other inventory positions sold, but not yet purchased | Derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	(8,439,000)	(223,000)	(16,751,000)	(5,873,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements				(745,000)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	10,846,000	254,000	6,643,000	5,218,000
Transfers in
Transfers out
Realized gains/(losses)	8,439,000	223,000	16,751,000	5,115,000
Unrealized gains/ (losses)	(679,000)	6,327,000	3,524,000	1,376,000
Ending balance	10,167,000	6,581,000	10,167,000	6,581,000
Financial instruments and other inventory positions owned
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	131,791,000	172,772,000	122,679,000	119,083,000
Purchases	58,028,000	99,763,000	132,378,000	448,986,000
Sales	(93,801,000)	(111,198,000)	(163,172,000)	(437,592,000)
Transfers in	515,000
Transfers out		(251,000)
Realized gains/ (losses)	1,377,000	12,900,000	7,140,000	32,819,000
Unrealized gains/ (losses)	718,000	(6,684,000)	(397,000)	4,006,000
Ending balance	98,628,000	167,302,000	98,628,000	167,302,000
Financial instruments and other inventory positions owned | Convertible securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		1,308,000
Purchases		0
Sales		(1,615,000)
Transfers in		0
Transfers out		(251,000)
Realized gains/ (losses)		839,000
Unrealized gains/ (losses)		(281,000)
Ending balance		0		0
Financial instruments and other inventory positions owned | Fixed income securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		100,000	100,000
Purchases		0	0	100,000
Sales		0	(100,000)
Transfers in		0	0
Transfers out		0	0
Realized gains/ (losses)		0	0
Unrealized gains/ (losses)		0	0
Ending balance	0	100,000	0	100,000
Financial instruments and other inventory positions owned | Tax-exempt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,306,000	1,433,000	1,433,000	1,429,000
Purchases				1,000
Sales
Transfers in
Transfers out
Realized gains/ (losses)				(266,000)
Unrealized gains/ (losses)			(127,000)	269,000
Ending balance	1,306,000	1,433,000	1,306,000	1,433,000
Financial instruments and other inventory positions owned | Short-term securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	732,000	656,000	656,000	656,000
Purchases
Sales			(25,000)
Transfers in
Transfers out
Realized gains/ (losses)			6,000
Unrealized gains/ (losses)	(1,000)		94,000
Ending balance	731,000	656,000	731,000	656,000
Financial instruments and other inventory positions owned | Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	129,126,000	151,396,000	119,799,000	116,171,000
Purchases	58,028,000	99,763,000	129,764,000	448,175,000
Sales	(93,801,000)	(97,246,000)	(163,047,000)	(424,274,000)
Transfers in	515,000
Transfers out
Realized gains/ (losses)	1,377,000	(276,000)	9,748,000	20,562,000
Unrealized gains/ (losses)	29,000	3,405,000	(990,000)	(3,592,000)
Ending balance	95,274,000	157,042,000	95,274,000	157,042,000
Financial instruments and other inventory positions owned | Derivative contracts
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			2,614,000	710,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		12,337,000		13,318,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	627,000	17,879,000	691,000	827,000
Transfers in
Transfers out
Realized gains/ (losses)		12,337,000	(2,614,000)	12,523,000
Unrealized gains/ (losses)	690,000	(9,808,000)	626,000	7,329,000
Ending balance	1,317,000	8,071,000	1,317,000	8,071,000
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	58,567,000	53,567,000	49,240,000	33,245,000
Purchases	10,500,000		20,500,000	15,363,000
Sales	(9,000)	(10,356,000)	(2,368,000)	(10,356,000)
Transfers in
Transfers out		(619,000)		(619,000)
Realized gains/ (losses)	9,000	5,942,000	2,368,000	5,946,000
Unrealized gains/ (losses)	931,000	(2,955,000)	258,000	2,000,000
Ending balance	$ 69,998,000	$ 45,579,000	$ 69,998,000	$ 45,579,000