Derivative Instruments - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Cash collateral received	$ 500	$ 800
Return of cash collateral into brokerage accounts	8,800	6,200
Losses on equity options activity	27,800	27,600
Exchange Traded Options [Member] | Long [Member]
Derivative [Line Items]
Cash collateral received	0	0
Long exchange-trade equity options at fair value	5,000	3,100
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	$ 13,600	$ 700