Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 5.2		$ 5.2		$ 1.0
Cash collateral in brokerage accounts	3.5		3.5		13.2
Losses on equity options activity	14.1	$ 24.5	54.5	$ 114.9
Exchange-traded options | Long
Derivative [Line Items]
Long exchange-traded equity options, classified as trading investments	11.2		11.2		22.3
Exchange-traded options | Short
Derivative [Line Items]
Short exchange-traded equity options, included in securities sold not yet purchased	$ 4.1		$ 4.1		$ 7.1