Derivative Instruments - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Cash collateral received	$ 4,800	$ 500
Return of cash collateral into brokerage accounts	4,500	8,800
Losses on equity options activity	7,900	27,800
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral received	0	0
Long exchange-trade equity options at fair value	2,600	5,000
Exchange-traded options | Short
Derivative [Line Items]
Return of cash collateral into brokerage accounts	0	0
Short exchange-traded equity options at fair value	$ 3,800	$ 13,600