Derivative Instruments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 0		$ 0		$ 800,000
Cash collateral received	6,400,000		6,400,000		6,200,000
Loss on equity options activity	12,700,000	$ 10,900,000	21,800,000	$ 27,600,000
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	9,400,000		9,400,000		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	$ 13,300,000		$ 13,300,000		$ 700,000