Derivative Instruments - Offseting Table (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Gross amount recognized on the balance sheet
Derivative assets and liabilities
Derivative Assets	$ 13.5	$ 15.3
Derivative Liabilities	46.5	12.5
Gross amount not offset on balance sheet
Derivative assets and liabilities
Derivative Assets	(5.0)	(0.4)
Derivative Liabilities	(19.2)	(11.5)
Cash collateral
Collateral in margin accounts	14.2	10.3
Net Amount
Derivative assets and liabilities
Derivative Assets	8.5	14.9
Derivative Liabilities	$ 27.3	$ 1.0