Derivative Instruments - Offsetting Table (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Offsetting Derivative Assets
Gross amount recognized on balance sheet	$ 12.0	$ 5.3
Gross amount not offset on the balance sheet	3.6	0.7
Net amount	8.4	4.6
Offsetting Derivative Liabilities
Gross amount recognized on the balance sheet	0.7	21.4
Gross amount not offset on the balance sheet	0.2	13.5
Net amount	0.5	7.9
Cash collateral
Cash collateral received	$ 3.4
Cash collateral posted		$ 12.8