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Goodwill
 9 Months Ended
Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]  
Goodwill Goodwill
Unless otherwise required, goodwill is tested for impairment annually in the fourth quarter. Changes in the carrying amount of goodwill for the nine months ended September 30, 2021 were (in millions):

Goodwill
Balance as of December 31, 2020
$375.3 
Acquisitions176.4 
Measurement period adjustments0.2 
Balance as of September 30, 2021
$551.9