Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 2,400		$ 500
Cash collateral received	4,700		8,800
Loss on equity options activity	4,200	$ 3,700
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0
Long exchange-traded equity options	5,400		5,000
Exchange-traded options | Short
Derivative [Line Items]
Cash collateral received	0		0
Short exchange-traded equity options, included in securities sold not yet purchased	$ 14,000		$ 13,600