Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 1,000		$ 1,000		$ 300
Cash collateral received	3,600		3,600		4,300
Loss on equity options activity	(4,400)	$ (3,600)	(4,500)	$ (14,700)
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	$ 11,100		$ 11,100		$ 12,300