Offsetting of Assets and Liabilities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Offsetting Assets and Liabilities [Line Items]
Collateral received	$ 8.7	$ 16.5
Gross amounts not offset in the balance sheet	1.1	0.1
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position	935.7	766.2
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position	922.7	327.1
Net maximum amount of termination payments that could have been required	13.0	439.1
Net Investment Hedges
Offsetting Assets and Liabilities [Line Items]
Collateral received	360.7
Gross amounts not offset in the balance sheet	$ 415.2
Cash collateral received from derivative counterparties and not offset against derivative assets		196.3
Cash collateral deposited with derivative counterparties not offset against derivative liabilities		$ 2.0