Derivative Instruments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 0		$ 0		$ 800,000
Cash collateral received	5,600,000		5,600,000		6,200,000
Loss on equity options activity	5,400,000	$ 8,100,000	9,200,000	$ 16,700,000
Exchange-traded options | Long
Derivative [Line Items]
Cash collateral payable to trade counterparties	0		0		0
Long exchange-traded equity options, classified as trading investments	8,300,000		8,300,000		3,100,000
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	$ 7,500,000		$ 7,500,000		$ 700,000