Goodwill and Intangible Assets - Change in the Carrying Amount of Goodwill by Segment (Details) $ in Millions	6 Months Ended
Jun. 30, 2021 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 8,485.2
Additions / opening balance sheet adjustments	214.2
Foreign currency impact	(46.2)
Ending balance	8,653.2
Freight Segment
Goodwill [Roll Forward]
Beginning balance	6,872.2
Additions / opening balance sheet adjustments	214.2
Foreign currency impact	2.8
Ending balance	7,089.2
Transit Segment
Goodwill [Roll Forward]
Beginning balance	1,613.0
Additions / opening balance sheet adjustments	0.0
Foreign currency impact	(49.0)
Ending balance	$ 1,564.0

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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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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