Offsetting of Assets and Liabilities (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Offsetting Assets and Liabilities [Line Items]
Collateral received	$ 2.8	$ 0.9
Gross amounts not offset in the balance sheet	0.5	0.5
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position	690.4	1,648.2
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position	689.0	1,044.0
Net maximum amount of termination payments that could have been required	1.4	604.2
Net Investment Hedges
Offsetting Assets and Liabilities [Line Items]
Collateral received	142.3
Gross amounts not offset in the balance sheet	$ 247.9
Cash collateral received from derivative counterparties and not offset against derivative assets		111.0
Cash collateral deposited with derivative counterparties not offset against derivative liabilities		$ 49.0