Derivative Instruments - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Cash collateral payable to trade counterparties	$ 1.5	$ 1.0
Return of cash collateral into brokerage accounts	12.8	13.2
Losses on equity options activity	(65.0)	(140.0)
Exchange Traded Options [Member] | Long [Member]
Derivative [Line Items]
Long exchange-trade equity options at fair value	5.9	22.3
Exchange Traded Options [Member] | Short [Member]
Derivative [Line Items]
Short exchange-traded equity options at fair value	$ 0.8	$ 7.1