Derivative Instruments - Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Cash collateral
Cash collateral posted in margin accounts	$ 34.4	$ 2.7
Cash collateral received in margin accounts		0.2
Offsetting derivative assets
Gross amount recognized on the balance sheet	6.9	15.9
Gross amount not offset on the balance sheet	(1.4)	(4.0)
Net amount	5.5	11.9
Cash collateral received		0.2
Offsetting derivative liabilities
Gross amount recognized on the balance sheet	28.9	7.9
Gross amount not offset on the balance sheet	(21.4)	(4.9)
Net amount	7.5	3.0
Cash collateral posted	$ 20.0	$ 1.1

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