Goodwill and Other Intangible Assets - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill	$ 1,198,600,000			$ 1,198,600,000		$ 1,260,100,000
Goodwill impairment charges	0			66,800,000	$ 64,000,000.0
Jefferson Wells
Goodwill [Roll Forward]
Goodwill	55,500,000			55,500,000
Right Management
Goodwill [Roll Forward]
Goodwill	62,100,000			62,100,000
Germany
Goodwill [Roll Forward]
Goodwill impairment charges		$ 66,800,000	$ 60,200,000
Americas | United States
Goodwill [Roll Forward]
Goodwill	490,200,000			490,200,000		490,300,000
Americas | United States | Jefferson Wells
Goodwill [Roll Forward]
Goodwill	545,700,000			545,700,000		545,800,000
Southern Europe | France
Goodwill [Roll Forward]
Goodwill	70,300,000			70,300,000		67,300,000
Southern Europe | Italy
Goodwill [Roll Forward]
Goodwill	$ 4,000,000.0			$ 4,000,000.0		$ 4,600,000