Derivative Instruments - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Cash collateral received	$ 800	$ 1,500
Return of cash collateral into brokerage accounts	6,200	12,800
Losses on equity options activity	27,600	65,000
Exchange Traded Options [Member] | Long [Member]
Derivative [Line Items]
Cash collateral received	0	0
Long exchange-trade equity options at fair value	3,100	5,900
Exchange Traded Options [Member] | Short [Member]
Derivative [Line Items]
Return of cash collateral into brokerage accounts	0	0
Short exchange-traded equity options at fair value	700	$ 800
Consolidated VIEs [Member]
Derivative [Line Items]
Cash collateral received	461
Return of cash collateral into brokerage accounts	3,311
Net derivative asset in VIE portfolios	2,900
Gain (loss) on derivative instruments	$ 800