Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, € in Millions	1 Months Ended				3 Months Ended					12 Months Ended
	Apr. 30, 2019 USD ($)	Apr. 30, 2019 EUR (€)	Apr. 30, 2018 USD ($)	Apr. 30, 2018 EUR (€)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) Office Country $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 31, 2019 shares	Mar. 31, 2019	Aug. 03, 2018 shares	Jul. 28, 2016 shares
Accounting Policies [Line Items]
Number of offices worldwide (over) | Office										2,200
Number of countries and territories | Country										75
Investments value										$ 106,600,000	$ 97,800,000	$ 161,400,000
Unremitted earning from investments										$ 8,000,000.0	3,400,000
Payment terms										60 days
Description of timing										Our client contracts are generally short-term in nature with a term of one year or less. The timing between satisfaction of the performance obligation, invoicing and payment is not significant.
Deferred revenue										$ 34,900,000	44,500,000
Advertising expenses										22,200,000	25,700,000	27,900,000
Restructuring costs										110,700,000	42,000,000.0	39,300,000
Operating lease right-of-use asset impairment										27,300,000
Costs paid, utilized or transferred										71,900,000	50,200,000
Reserve will be paid by end of period										46,100,000	7,300,000	15,500,000
Consolidated amortization expense related to intangibles										27,200,000	29,800,000	35,100,000
2021										16,800,000
2022										13,400,000
2023										10,700,000
2024										8,400,000
2025										6,300,000
Non-cash impairment charge loss					$ 0		$ 0		$ 0	66,800,000	64,000,000.0
Goodwill										1,225,800,000	1,260,100,000	1,297,100,000
Percentage interest acquired																			51.00%
Percentage of ownership in Swiss franchise																			49.00%
Swiss franchise investment portfolio market value												219,900,000
Swiss franchise realized gains										0.0	300,000	12,700,000
Swiss franchise realized losses										0.0	200,000	2,100,000
Net capitalized software balance										19,200,000	7,500,000
Amortization expense related to capitalized software cost										$ 1,800,000	$ 2,000,000.0	$ 1,500,000
Shares authorized to be repurchased (in shares) | shares																		6,000,000.0		6,000,000.0	6,000,000.0
Shares repurchased (in shares) | shares										3,400,000	2,400,000	5,700,000
Total cost of shares repurchased										$ 264,700,000	$ 203,000,000.0	$ 500,700,000
Dividends declared (in dollars per share) | $ / shares										$ 2.26	$ 2.18	$ 2.02
Total dividend payments										$ 129,100,000	$ 129,300,000	$ 127,300,000
Net earnings, net of tax, attributable to noncontrolling interests										$ 39,900,000	(40,600,000)	$ 42,000,000.0
CICE payroll tax credit												6.00%	7.00%	6.00%	6.00%	6.00%	4.00%
Multiplier for payroll tax credits										2.5
Payment period										3 years
Net proceeds from sale of tax credits	$ 103,500,000	€ 92.0	$ 234,500,000	€ 190.9
Accounting Standards Update 2016-13
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted										true
Change in accounting principle, accounting standards update, adoption date										Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect										true
Accounting Standards Update 2018-13
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted										true
Change in accounting principle, accounting standards update, adoption date										Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect										true
Accounting Standards Update 2018-15
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted										true
Change in accounting principle, accounting standards update, adoption date										Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect										true
Accounting Standards Update 2018-14
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted										true
Change in accounting principle, accounting standards update, adoption date										Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect										true
Noncontrolling Interests
Accounting Policies [Line Items]
Net earnings, net of tax, attributable to noncontrolling interests										$ 4,700,000	$ (1,800,000)	$ 4,900,000
August 2019 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares										2,600,000	2,400,000
Shares remaining authorized for repurchase (in shares) | shares										3,400,000
August 2018 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares										800,000		2,900,000
Shares remaining authorized for repurchase (in shares) | shares										0
July 2016 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares												2,800,000
Shares remaining authorized for repurchase (in shares) | shares										0
Leasehold Improvements
Accounting Policies [Line Items]
Estimated useful lives										lesser of life of asset or expected lease term
United Kingdom
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit										12.00%
United Kingdom | Discount Rates
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										11.5
United Kingdom | Terminal Value Revenue Growth Rate
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										1.0
United Kingdom | Terminal Value OUP Margin
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										3.1
Netherlands
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit										3.30%
Netherlands | Discount Rates
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										10.9
Netherlands | Terminal Value Revenue Growth Rate
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										2.0
Netherlands | Terminal Value OUP Margin
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)										3.5
Northern Europe | United Kingdom
Accounting Policies [Line Items]
Goodwill										$ 100,200,000	$ 97,300,000
Northern Europe | Netherlands
Accounting Policies [Line Items]
Goodwill										$ 119,300,000	109,500,000
Northern Europe | Germany
Accounting Policies [Line Items]
Non-cash impairment charge loss						$ 66,800,000		$ 60,200,000
Goodwill											$ 67,200,000
APME | New Zealand
Accounting Policies [Line Items]
Non-cash impairment charge loss								$ 3,800,000
Minimum
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit										20.00%
Remaining lease terms										1 month
Minimum | Furniture, Fixtures, and Autos
Accounting Policies [Line Items]
Estimated useful lives										2 years
Minimum | Computer Equipment
Accounting Policies [Line Items]
Estimated useful lives										2 years
Minimum | Discount Rates
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)					0.091
Maximum
Accounting Policies [Line Items]
Remaining lease terms										11 years
Maximum | Buildings
Accounting Policies [Line Items]
Estimated useful lives										40 years
Maximum | Furniture, Fixtures, and Autos
Accounting Policies [Line Items]
Estimated useful lives										15 years
Maximum | Computer Equipment
Accounting Policies [Line Items]
Estimated useful lives										15 years
Maximum | Discount Rates
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)					0.135
Maximum | Netherlands
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit										10.00%
Customer relationships
Accounting Policies [Line Items]
Useful life										12 years
Other
Accounting Policies [Line Items]
Useful life										3 years
Computer software | Minimum
Accounting Policies [Line Items]
Useful life											3 years
Computer software | Maximum
Accounting Policies [Line Items]
Useful life											10 years
Euro-denominated notes | Estimate of fair value measurement | Significant Other Observable Inputs (Level 2)
Accounting Policies [Line Items]
Carrying value of long-term debt										$ 1,094,500,000	$ 1,062,500,000
Euro-denominated notes | Carrying value | Significant Other Observable Inputs (Level 2)
Accounting Policies [Line Items]
Carrying value of long-term debt										1,159,100,000	1,002,900,000
Allowance for doubtful accounts
Accounting Policies [Line Items]
Bad debt expense										20,300,000	21,800,000	$ 23,000,000.0
Write-offs										$ 17,800,000	$ 19,100,000	$ 12,000,000.0