Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 300		$ 300		$ 4,800
Cash collateral received	6,900		6,900		4,500
Loss on equity options activity	3,500	$ 4,700	11,100	$ 8,900
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options	2,300		2,300		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	$ 2,200		$ 2,200		$ 3,800