Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 1,800		$ 1,500
Cash collateral in brokerage accounts	11,200		12,800
Losses on equity options activity	8,600	$ 32,800
Net gain on derivative instruments	(3,557)	$ (6,055)
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0
Long exchange-traded equity options, classified as trading investments	4,700		5,900
Exchange-traded options | Short
Derivative [Line Items]
Cash collateral in brokerage accounts	0		0
Short exchange-traded equity options, included in securities sold not yet purchased	3,300		$ 800
Consolidated VIEs
Derivative [Line Items]
Cash collateral payable to trade counterparties	1,460
Cash collateral in brokerage accounts	9,391
Net derivative asset in VIE portfolios	400
Net gain on derivative instruments	$ 3,000