Taxation (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Credit Carryforward [Line Items]
Income tax expense (benefit)	$ 13.1	$ (130.7)
Tax loss carryforwards	$ 402.4	$ 413.3
Statutory tax rate (percent)	21.00%	35.00%	35.00%
Unremitted foreign earnings	$ 935.0	$ 988.0
Deferred tax liabilities arising from unremitted earnings	0.1	1.4
Unrecognized tax benefits	20.0	19.6	$ 10.5	$ 9.6
Unrecognized tax benefits that would impact effective tax rate	16.4	16.5	9.5
Income tax provision	$ 255.0	268.2	338.3
U.K
Tax Credit Carryforward [Line Items]
Statutory tax rate (percent)	19.00%
Canada
Tax Credit Carryforward [Line Items]
Dividends withholding tax rate (percent)	5.00%
Related to accrued interest and penalties [Member]
Tax Credit Carryforward [Line Items]
Income Tax Examination, Penalties and Interest Accrued	$ 3.2	2.9	1.7
Income tax provision	0.3	$ 1.2	$ 0.5
Minimum
Tax Credit Carryforward [Line Items]
Tange of possible changes in unrecognized tax benefitsper Bound	0.0
Maximum
Tax Credit Carryforward [Line Items]
Tange of possible changes in unrecognized tax benefitsper Bound	10.0
Expiring Ibetween 2019 and 2021
Tax Credit Carryforward [Line Items]
Tax loss carryforward subject to expiration	35.4
Expiring after 2021
Tax Credit Carryforward [Line Items]
Tax loss carryforward subject to expiration	14.0
Indefinite life
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	353.0
Change during the period
Tax Credit Carryforward [Line Items]
Increase (decrease) in tax loss carryforwards	(10.9)
Operating Loss Carryforwards resulting from acquisitions
Tax Credit Carryforward [Line Items]
Increase (decrease) in tax loss carryforwards	21.8
Operating Loss Carryforwards resulting from FX
Tax Credit Carryforward [Line Items]
Increase (decrease) in tax loss carryforwards	(10.4)
General Business Tax Credit Carryforward
Tax Credit Carryforward [Line Items]
Increase (decrease) in tax loss carryforwards	$ (24.0)