Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 01, 2018
Goodwill
Goodwill	$ 4,945.5	$ 4,894.4	$ 4,875.9	$ 1,110.8
Current year acquisitions	24.0	2.8
Foreign currency and other	27.1	15.7
IAN
Goodwill
Goodwill	4,264.5	4,218.1	4,201.5
Current year acquisitions	23.5	2.1
Foreign currency and other	22.9	14.5
DXTRA
Goodwill
Goodwill	681.0	676.3	$ 674.4
Current year acquisitions	0.5	0.7
Foreign currency and other	$ 4.2	$ 1.2

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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