Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 800		$ 800		$ 1,500
Cash collateral in brokerage accounts	5,900		5,900		12,800
Losses on equity options activity	8,100	$ 7,600	16,700	$ 40,400
Gain (loss) on derivative instruments	(5,052)	$ 2,473	(8,609)	$ (3,582)
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	8,000		8,000		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	800		800		$ 800
Consolidated VIEs
Derivative [Line Items]
Cash collateral payable to trade counterparties	422		422
Cash collateral in brokerage accounts	9,493		9,493
Net derivative asset in VIE portfolios	1,500		1,500
Gain (loss) on derivative instruments	$ (300)		$ 2,700