Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 300		$ 300		$ 500
Cash collateral received	3,000		3,000		8,800
Loss on equity options activity	4,700	$ 5,400	8,900	$ 9,200
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options	8,200		8,200		5,000
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	$ 18,300		$ 18,300		$ 13,600