Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($)	3 Months Ended				12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Apr. 03, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill					$ 1,260,100,000		$ 1,297,100,000		$ 1,343,000,000.0
Goodwill, accumulated impairment loss					577,400,000		513,400,000	$ 513,400,000
Impairment charge	$ 0		$ 0	$ 0	64,000,000.0
Corporate
Goodwill [Roll Forward]
Goodwill					63,900,000		65,500,000		$ 65,500,000
Goodwill, accumulated impairment loss					278,200,000		278,200,000	278,200,000
Jefferson Wells | Corporate
Goodwill [Roll Forward]
Goodwill					55,500,000
Americas | United States
Goodwill [Roll Forward]
Goodwill					490,300,000		476,500,000	476,500,000
Americas | United States | Jefferson Wells
Goodwill [Roll Forward]
Goodwill					545,800,000		532,000,000.0
Southern Europe
Goodwill [Roll Forward]
Goodwill						$ 33,800,000
Southern Europe | France
Goodwill [Roll Forward]
Goodwill					67,300,000		68,900,000	76,300,000
Southern Europe | Italy
Goodwill [Roll Forward]
Goodwill					4,600,000		4,800,000	5,000,000.0
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					60,200,000
Northern Europe | Germany
Goodwill [Roll Forward]
Goodwill					67,200,000		$ 129,200,000
Impairment charge		$ 60,200,000
APME
Goodwill [Roll Forward]
Goodwill, accumulated impairment loss					$ 3,800,000
APME | New Zealand
Goodwill [Roll Forward]
Impairment charge		$ 3,800,000