Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($)	3 Months Ended					12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Apr. 03, 2019	Jan. 01, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill						$ 1,225,800,000	$ 1,260,100,000			$ 1,297,100,000
Goodwill, accumulated impairment loss						644,200,000	577,400,000		$ 513,400,000
Goodwill impairment charges	$ 0		$ 0		$ 0	66,800,000	64,000,000.0
Corporate
Goodwill [Roll Forward]
Goodwill						126,000,000.0	126,000,000.0			$ 127,600,000
Goodwill, accumulated impairment loss						278,200,000	278,200,000		278,200,000
Jefferson Wells
Goodwill [Roll Forward]
Goodwill						55,500,000	55,500,000
Right Management
Goodwill [Roll Forward]
Goodwill						62,100,000	62,100,000
Americas | United States
Goodwill [Roll Forward]
Goodwill						490,200,000	490,300,000		490,300,000
Americas | United States | Jefferson Wells
Goodwill [Roll Forward]
Goodwill						545,700,000	545,800,000
Southern Europe
Goodwill [Roll Forward]
Goodwill								$ 33,800,000
Southern Europe | France
Goodwill [Roll Forward]
Goodwill						73,300,000	67,300,000		68,900,000
Southern Europe | Italy
Goodwill [Roll Forward]
Goodwill						4,200,000	4,600,000		4,800,000
Right Management
Goodwill [Roll Forward]
Goodwill, accumulated impairment loss						235,200,000	235,200,000		$ 235,200,000
Northern Europe
Goodwill [Roll Forward]
Goodwill, accumulated impairment loss						127,000,000.0	60,200,000
Northern Europe | Germany
Goodwill [Roll Forward]
Goodwill							67,200,000
Goodwill impairment charges		$ 66,800,000		$ 60,200,000
APME
Goodwill [Roll Forward]
Goodwill, accumulated impairment loss						$ 3,800,000	$ 3,800,000
APME | New Zealand
Goodwill [Roll Forward]
Goodwill impairment charges				$ 3,800,000