Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 800		$ 800		$ 300
Cash collateral received	5,900		5,900		4,300
Loss on equity options activity	(18,500)	$ (3,500)	(100)	$ (11,100)
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options	12,200		12,200		6,900
Exchange-traded options | Short
Derivative [Line Items]
Cash collateral received	0		0		0
Short exchange-traded equity options, included in securities sold not yet purchased	$ 9,400		$ 9,400		$ 12,300