Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 300		$ 300		$ 4,800
Cash collateral received	1,900		1,900		4,500
Gain (loss) on equity options activity	(3,600)	$ 5,700	(14,700)	$ (3,200)
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options	5,800		5,800		2,600
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	$ 5,200		$ 5,200		$ 3,800