Derivative Financial Instruments - Schedule of Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets, fair value gross	$ 109.5	$ 80.4
Cash collateral	9.6	12.5
Total fair value of gross derivative assets	99.9	67.9
Derivative liabilities, fair value, gross	112.9	68.3
Cash collateral	73.1	10.9
Total fair value of gross derivative liabilities	39.8	57.4
Derivative assets	68.8	39.0
Total net derivative assets	59.2	26.5
Derivative liabilities	72.1	26.9
Total net derivative liabilities	$ (1.0)	16.0
Over the counter
Derivative [Line Items]
Derivative assets, fair value gross		80.4
Derivative liabilities, fair value, gross		68.2
Derivative assets		39.0
Derivative liabilities		26.8
Exchange traded
Derivative [Line Items]
Derivative assets, fair value gross		0.0
Derivative liabilities, fair value, gross		0.1
Derivative assets		0.0
Derivative liabilities		$ 0.1