Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) - USD ($)	3 Months Ended			12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Balance at beginning of period				$ 1,297,100,000	$ 1,343,000,000.0
Goodwill acquired during the period				48,000,000.0	6,100,000
Goodwill allocated to business units sold/deconsolidated				(20,100,000)	(8,800,000)
Impairment charge	$ 0	$ 0	$ 0	(64,000,000.0)
Currency impact and other				(900,000)	(43,200,000)
Balance at end of period				1,260,100,000	1,297,100,000
Reportable segments | Americas
Goodwill [Roll Forward]
Balance at beginning of period				519,900,000	519,200,000
Goodwill acquired during the period				14,200,000	4,600,000
Currency impact and other				1,500,000	(3,900,000)
Balance at end of period				535,600,000	519,900,000
Reportable segments | Southern Europe
Goodwill [Roll Forward]
Balance at beginning of period				112,200,000	121,900,000
Goodwill acquired during the period				33,800,000
Currency impact and other				(1,200,000)	(9,700,000)
Balance at end of period				144,800,000	112,200,000
Reportable segments | Northern Europe
Goodwill [Roll Forward]
Balance at beginning of period				435,400,000	468,100,000
Goodwill allocated to business units sold/deconsolidated					(8,800,000)
Impairment charge				(60,200,000)
Currency impact and other				(600,000)	(23,900,000)
Balance at end of period				374,600,000	435,400,000
Reportable segments | APME
Goodwill [Roll Forward]
Balance at beginning of period				102,000,000.0	106,200,000
Goodwill acquired during the period					1,500,000
Goodwill allocated to business units sold/deconsolidated				(18,500,000)
Impairment charge				(3,800,000)
Currency impact and other				(600,000)	(5,700,000)
Balance at end of period				79,100,000	102,000,000.0
Reportable segments | Right Management
Goodwill [Roll Forward]
Balance at beginning of period				62,100,000	62,100,000
Balance at end of period				62,100,000	62,100,000
Corporate
Goodwill [Roll Forward]
Balance at beginning of period				65,500,000	65,500,000
Goodwill allocated to business units sold/deconsolidated				(1,600,000)
Balance at end of period				$ 63,900,000	$ 65,500,000