Derivative Instruments - Balance Sheeet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Cash collateral
Collateral in margin accounts	$ 16.4	$ 42.3
Collateral received	4.4
Offsetting Derivative Assets
Gross amount recognized on the balance sheet	41.6	9.9
Gross amount not offset on the balance sheet	4.9	3.0
Derivative Asset	36.7	6.9
Cash collateral received	4.4
Offsetting Derivative Liabilities
Gross amount recognized on balance sheet	2.4	59.0
Gross amount not offset on the balance sheet	0.5	22.5
Derivative Liability	1.9	36.5
Cash collateral posted		19.5
Derivative, Credit Risk Related Contingent Features [Abstract]
Aggregate fair value of derivative instruments with credit risk-related contingent features that were in a liability position	$ 0.2	23.8
Collateral that would have been required		$ 18.0