Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 800		$ 800		$ 1,500
Cash collateral in brokerage accounts	5,100		5,100		12,800
Losses on equity options activity	10,900	$ 14,100	27,600	$ 54,500
Gain (loss) on derivative instruments	(8,042)	$ 23,733	(16,651)	$ 20,151
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options, classified as trading investments	3,500		3,500		5,900
Exchange-traded options | Short
Derivative [Line Items]
Cash collateral in brokerage accounts	0		0		0
Short exchange-traded equity options, included in securities sold not yet purchased	4,300		4,300		$ 800
Consolidated VIEs
Derivative [Line Items]
Cash collateral payable to trade counterparties	393		393
Cash collateral in brokerage accounts	3,016		3,016
Net derivative asset in VIE portfolios	300		300
Gain (loss) on derivative instruments	$ (2,000)		$ 700