Segment Data - Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services	$ 5,393.2	$ 5,418.7	$ 5,656.9	$ 5,522.4	$ 5,637.5	$ 5,464.8	$ 5,174.8	$ 4,757.2	$ 21,991.2	$ 21,034.3	$ 19,654.1
Operating Unit Profit									941.8	932.2	874.3
Corporate expenses									(110.0)	(108.4)	(92.8)
Intangible asset amortization expense									(35.1)	(34.6)	(36.0)
Interest and other expenses									(42.0)	(51.9)	(44.2)
Earnings before income taxes									754.7	737.3	701.3
Depreciation and Amortization Expense									85.8	84.4	85.3
Earnings from Equity Investments									1.2	15.0	3.6
Total Assets	8,519.8				8,883.6				8,519.8	8,883.6	7,574.2
Equity Investments	161.4				158.7				161.4	158.7	145.8
Long-Lived Assets	160.0				161.3				160.0	161.3	150.6
Additions to Long-Lived Assets									60.3	53.9	54.6
Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									4,159.3	4,216.4	4,297.2
Operating Unit Profit									203.9	213.3	195.9
Earnings from Equity Investments									0.0	0.0	0.0
Americas | United States
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									2,522.3	2,659.0	2,836.8
Operating Unit Profit									130.8	152.1	142.3
Earnings from Equity Investments									0.0	0.0	0.0
Americas | Other Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									1,637.0	1,557.4	1,460.4
Operating Unit Profit									73.1	61.2	53.6
Earnings from Equity Investments									0.0	0.0	0.0
Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									9,371.6	8,657.0	7,497.6
Operating Unit Profit									467.6	443.9	378.8
Earnings from Equity Investments									1.5	15.0	3.6
Southern Europe | France
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									5,827.7	5,477.2	4,837.4
Operating Unit Profit									290.4	280.0	252.5
Earnings from Equity Investments									0.0	0.0	0.0
Southern Europe | Italy
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									1,670.6	1,475.9	1,167.7
Operating Unit Profit									111.1	104.5	79.1
Earnings from Equity Investments									(0.2)	0.0	0.0
Southern Europe | Other Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									1,873.3	1,703.9	1,492.5
Operating Unit Profit									66.1	59.4	47.2
Earnings from Equity Investments									1.7	15.0	3.6
Northern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									5,370.5	5,306.4	5,129.1
Operating Unit Profit									122.7	140.1	166.4
Earnings from Equity Investments									0.0	0.0	0.0
APME
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									2,890.3	2,636.4	2,471.3
Operating Unit Profit									114.8	98.9	88.5
Earnings from Equity Investments									(0.3)	0.0	0.0
Right Management
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues from Services									199.5	218.1	258.9
Operating Unit Profit									32.8	36.0	44.7
Earnings from Equity Investments									0.0	0.0	0.0
Reportable segments | Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									10.4	11.8	12.6
Total Assets	2,168.9				2,110.6				2,168.9	2,110.6	2,033.3
Equity Investments	0.0				0.0				0.0	0.0	0.0
Long-Lived Assets	25.6				26.7				25.6	26.7	34.0
Reportable segments | Americas | United States
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									8.2	9.3	9.9
Total Assets	1,827.4				1,781.4				1,827.4	1,781.4	1,718.9
Equity Investments	0.0				0.0				0.0	0.0	0.0
Long-Lived Assets	19.6				20.6				19.6	20.6	27.7
Reportable segments | Americas | Other Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									2.2	2.5	2.7
Total Assets	341.5				329.2				341.5	329.2	314.4
Equity Investments	0.0				0.0				0.0	0.0	0.0
Long-Lived Assets	6.0				6.1				6.0	6.1	6.3
Reportable segments | Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									20.7	18.8	16.4
Total Assets	3,711.4				3,786.0				3,711.4	3,786.0	2,889.8
Equity Investments	158.1				157.6				158.1	157.6	139.7
Long-Lived Assets	77.6				77.1				77.6	77.1	62.4
Reportable segments | Southern Europe | France
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									14.0	12.3	11.0
Total Assets	2,729.7				2,753.1				2,729.7	2,753.1	2,104.8
Equity Investments	0.0				0.0				0.0	0.0	0.2
Long-Lived Assets	49.2				47.9				49.2	47.9	39.7
Reportable segments | Southern Europe | Italy
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									1.9	1.8	1.9
Total Assets	405.0				436.7				405.0	436.7	294.9
Equity Investments	0.3				0.4				0.3	0.4	0.4
Long-Lived Assets	5.0				4.9				5.0	4.9	4.4
Reportable segments | Southern Europe | Other Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									4.8	4.7	3.5
Total Assets	576.7				596.2				576.7	596.2	490.1
Equity Investments	157.8				157.2				157.8	157.2	139.1
Long-Lived Assets	23.4				24.3				23.4	24.3	18.3
Reportable segments | Northern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									11.0	10.6	10.9
Total Assets	1,237.0				1,569.0				1,237.0	1,569.0	1,292.4
Equity Investments	0.0				0.0				0.0	0.0	0.1
Long-Lived Assets	29.9				28.0				29.9	28.0	25.4
Reportable segments | APME
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									5.7	4.7	5.3
Total Assets	754.0				780.7				754.0	780.7	612.8
Equity Investments	2.3				0.1				2.3	0.1	0.0
Long-Lived Assets	21.5				21.4				21.5	21.4	17.9
Reportable segments | Right Management
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									2.8	3.7	3.9
Total Assets	127.6				138.1				127.6	138.1	136.6
Equity Investments	0.0				0.0				0.0	0.0	0.0
Long-Lived Assets	5.3				8.0				5.3	8.0	10.7
Corporate
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Depreciation and Amortization Expense									0.1	0.2	0.2
Total Assets	520.9				499.2				520.9	499.2	609.3
Equity Investments	1.0				1.0				1.0	1.0	6.0
Long-Lived Assets	$ 0.1				$ 0.1				0.1	0.1	0.2
Additions to Long-Lived Assets									0.1	0.0	0.0
Segment Reconciling Items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Intangible asset amortization expense									(35.1)	(34.6)	(36.0)
Segment Reconciling Items | Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									10.4	9.1	13.8
Segment Reconciling Items | Americas | United States
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									7.6	6.7	11.9
Segment Reconciling Items | Americas | Other Americas
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									2.8	2.4	1.9
Segment Reconciling Items | Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									26.2	25.7	23.9
Segment Reconciling Items | Southern Europe | France
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									18.1	15.2	13.3
Segment Reconciling Items | Southern Europe | Italy
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									2.2	1.7	1.7
Segment Reconciling Items | Southern Europe | Other Southern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									5.9	8.8	8.9
Segment Reconciling Items | Northern Europe
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									16.6	11.8	8.5
Segment Reconciling Items | APME
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									6.8	6.3	3.9
Segment Reconciling Items | Right Management
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Additions to Long-Lived Assets									$ 0.2	$ 1.0	$ 4.5