Offsetting of Assets and Liabilities (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Collateral received	$ 10.2	$ 2.1
Gross Amounts Not Offset in the Balance Sheet	3.7
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position	307.5	223.7
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position	230.7	35.8
Net maximum amount of termination payments that could have been required	76.8	187.9
Net Investment Hedges
Offsetting Assets and Liabilities [Line Items]
Collateral received	136.5
Gross Amounts Not Offset in the Balance Sheet	$ 145.7
Cash collateral received from derivative counterparties and not offset against derivative assets		67.0
Cash collateral deposited with derivative counterparties not offset against derivative liabilities		$ 143.1