Derivative Instruments - Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross amount recognized on the balance sheet	$ 15.9	$ 12.4
Gross amount not offset on the balance sheet	(4.0)	(4.9)
Net amount	11.9	7.5
Cash collateral received	0.2
Offsetting Derivative Liabilities
Gross amount recognized on balance sheet	7.9	11.6
Gross amount not offset on the balance sheet	(4.9)	(9.0)
Net amount	3.0	2.6
Cash collateral posted	1.1	4.1
Cash collateral
Cash collateral in margin account	2.7	16.2
Cash collateral received	$ 0.2
Credit Risk Related Contingent Features
Aggregate fair value of derivative instruments with credit risk-related contingent features that were in a liability position		3.7
Collateral that would have been required		$ 2.7