Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
Effective Income Tax Rate Reconciliation, Reorganizations, Amount	$ 30.5
One-time tax reserves		$ 256.9	$ 98.7
Deferred tax assets with full valuation allowance	635.0
Deferred tax asset, valuation allowance, excluding de-valued deferred tax assets	2.6
Valuation allowance recorded against deferred tax assets	4.3
Deferred tax assets, tax credit carryforwards, excluding adoption of ASU 2013-11	119.0
Tax credit carryforward, valuation allowance	118.6
Foreign tax credit carryforwards	87.6
Tax credit carryforwards, research	22.4
Tax credit carryforward	8.5
Deferred tax assets, net operating loss	1,990.0
Operating loss carryforwards, valuation allowance	1,902.8
Operating loss carryforwards subject to offset in accordance with ASU 2013--11	21.9
Tax loss and deduction carryforwards	1,997.5	2,111.5
Valuation allowance	2,327.6	2,960.0
Decrease in deferred tax liability associated with undistributed earnings of foreign subsidiaries	4.6
Undistributed earnings of foreign subsidiaries	1,500.0
Decrease in deferred tax liability associated with undistributed earnings of foreign subsidiaries	6.3
Total gross unrecognized tax benefits	372.1	331.7	137.6	$ 48.6
Unrecognized tax benefits that would impact effective tax rate	106.4
Expense (reversal of expense) for interest and penalties	1.0	(1.0)	(1.0)
Accrued interest and penalties	6.8	6.3
United Kingdom Tax Authority
Tax Credit Carryforward [Line Items]
Foreign income tax benefit	21.0
Foreign
Tax Credit Carryforward [Line Items]
Operating loss carryforwards	8,289.7
Deferred tax asset, not subject to expiration	6,938.7
Deferred tax assets, operating loss carryforwards, subject to expiration	1,351.0
Domestic
Tax Credit Carryforward [Line Items]
Tax loss and deduction carryforwards	29.5
Deferred tax asset, not subject to expiration	140.5
Deferred Tax Assets That Cannot Be Benefitted
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	65.1
Deferred Tax Assets Generated In Current Year
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	69.9
Deferred Tax Asset, Tax On Foreign Income
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	21.0
Deferred Tax Assets, Change In Judgment
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	4.7
Deferred Tax Assets, True-up Adjustment
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	(30.5)
Deferred Tax Assets Utilization Of Foreign Tax Credits
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance		(232.5)
Deferred Tax Assets Tax Rate Change And Write Off
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance		$ (66.5)
Deferred Tax Asset Generated In Current Year
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			138.6
Deferred Tax Assets Expensed Or Written Off In Current Year
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			93.0
Excess Tax Basis Reallocated
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			57.2
State Deferred Tax Assets
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			15.3
Other Deferred Tax Assets
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			11.7
Deferred Tax Assets Associated With Repatriation Of Earnings
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance			$ 8.8
Deferred Tax Asset Associated With Merger
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	81.8
Deferred tax assets with full valuation allowance	465.0
Deferred Tax Asset Realization Remote
Tax Credit Carryforward [Line Items]
Deferred tax assets with full valuation allowance	170.0
Deferred Tax Asset Realization Remote | Foreign
Tax Credit Carryforward [Line Items]
Deferred tax assets with full valuation allowance	70.0
Deferred Tax Asset Realization Remote | State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
Deferred tax assets with full valuation allowance	100.0
Deferred Tax Asset, Currency Translation And Other
Tax Credit Carryforward [Line Items]
Net increase (decrease) in valuation allowance	19.4
Valuation Allowance, Federal Loss Carryforwards | Domestic
Tax Credit Carryforward [Line Items]
Valuation allowance	$ 29.5