Taxation (Operating Loss and Tax Credit Carryforwards, and Their Expiration Dates) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Within 1 year	$ 1.4
Within 1 to 2 years	34.4
Within 2 to 3 years	18.4
Within 3 to 4 years	44.3
Within 4 to 5 years	50.1
Within 5 to 6 years	31.8
After 6 years	919.5
Indefinitely	587.1
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	1,687.0	$ 1,907.3
U.S. federal tax
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	489.6	687.1
U.S. state tax
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	140.3	170.7
Republic of Ireland
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	29.4	45.1
Foreign tax jurisdictions
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	723.8	614.9
R&D and other tax credits
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 303.9	$ 389.5