Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 1.8		$ 1.8		$ 1.0
Cash collateral in brokerage accounts	6.4		6.4		13.2
Losses on equity options activity	7.6	$ 29.6	40.4	$ 90.4
Exchange Traded Options [Member] | Long [Member]
Derivative [Line Items]
Long exchange-traded equity options, classified as trading investments	7.9		7.9		22.3
Exchange Traded Options [Member] | Short [Member]
Derivative [Line Items]
Short exchange-traded equity options, included in securities sold not yet purchased	$ 1.9		$ 1.9		$ 7.1