Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($)	3 Months Ended					12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2020	Jan. 01, 2019
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						$ 1,722,200,000	$ 1,225,800,000	$ 1,260,100,000
Goodwill, accumulated impairment loss						644,200,000	644,200,000		$ 577,400,000
Goodwill impairment charges	$ 0	$ 0		$ 0		0	66,800,000	64,000,000.0
Corporate
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						126,000,000.0	126,000,000.0	$ 126,000,000.0
Goodwill, accumulated impairment loss						278,200,000	278,200,000		278,200,000
Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						55,500,000	55,500,000
Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						62,100,000	62,100,000
Goodwill, accumulated impairment loss						235,200,000	235,200,000			$ 235,200,000
United States | Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						1,091,700,000	545,700,000
Americas | United States
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						1,013,000,000.0	490,200,000		490,300,000
Southern Europe | France
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						74,300,000	73,300,000
Southern Europe | France | Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						68,200,000	73,300,000		67,300,000
Southern Europe | Italy
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill						3,900,000	4,200,000		4,600,000
Northern Europe
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill, accumulated impairment loss						127,000,000.0	127,000,000.0		60,200,000
Northern Europe | Germany
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill impairment charges			$ 66,800,000		$ 60,200,000
APME
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill, accumulated impairment loss						$ 3,800,000	$ 3,800,000		$ 3,800,000
APME | New Zealand
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill impairment charges					$ 3,800,000