Taxation (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Unrecognized tax benefits	$ 70.0	$ 20.0
Additions for tax positions related to acquisitions	54.1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	12.7
Other reductions for tax positions related to prior years	5.3
Tax indemnification asset	22.1
Minimum
Income Tax Contingency [Line Items]
Range of possible change in gross unrecognized tax benefits	5.0
Maximum
Income Tax Contingency [Line Items]
Range of possible change in gross unrecognized tax benefits	15.0
OppenheimerFunds
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 9.5