Significant Accounting Policies - Textual (Details)								3 Months Ended			12 Months Ended
	May 01, 2024	Oct. 01, 2017	Apr. 01, 2017	May 03, 2016	Oct. 23, 2014 USD ($)	Mar. 20, 2013 USD ($)	Mar. 28, 2012	Dec. 31, 2017 USD ($) Clients	Dec. 31, 2016 USD ($) Clients	Sep. 30, 2016 USD ($) reportable_segment	Dec. 31, 2017 USD ($) Clients investment $ / shares	Dec. 31, 2016 USD ($) Clients investment $ / shares shares	Dec. 31, 2015 USD ($) Clients $ / shares shares	Mar. 23, 2016	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Accretion on Convertible Preferred Shares											$ (6,352,000)	$ 0	$ 0
Earnings Per Share, Increase (Decrease) Due To Non-Controlling Dividend As A Result Of Redeemable Redemption Value In Excess of Fair Value | $ / shares											$ 0	$ 0	$ 0
Long-Term Growth Rate Assumed in Annual Goodwill Impairment Test		3.00%
Debt Issuance Costs, Net								$ 17,587,000	$ 18,420,000		$ 17,587,000	$ 18,420,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability											$ (4,819,000)	$ 8,227,000	$ 38,887,000
Clients exceeding consolidated accounts receivable percentage | Clients								0	0		0	0
Number of Reporting Units											32
Consolidated accounts receivable percentage								10.00%	10.00%		10.00%	10.00%
Clients exceeding consolidated largest client revenue | Clients											0	0	0
Consolidated largest client revenue											10.00%	10.00%	10.00%
Distributions from non-consolidated affiliates											$ 3,672,000	$ 7,402,000	$ 0
Cost method investments								$ 9,527,000	$ 10,132,000		9,527,000	10,132,000
Goodwill, Impairment Loss											3,238,000	48,524,000
Business combination, acquisition related costs											877,000	2,640,000	$ 2,912,000
Assets								1,698,892,000	1,577,378,000		1,698,892,000	1,577,378,000
Liabilities								$ 1,791,519,000	2,026,674,000		1,791,519,000	$ 2,026,674,000
Stock Issued During Period, Shares, New Issues | shares												0	0
Pension expense											$ 10,124,000	$ 10,026,000	$ 6,731,000
Doner
Significant Accounting Policies [Line Items]
Variable interest entity, qualitative or quantitative information, ownership percentage			65.00%
Minimum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life											3 years
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount								12.00%			12.00%
Weighted Average Cost Of Capital, Annual Goodwill Impairment Test		9.67%
Maximum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life											7 years
Equity method investment, ownership percentage								50.00%			50.00%
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount								100.00%			100.00%
Weighted Average Cost Of Capital, Annual Goodwill Impairment Test		11.85%
Doner
Significant Accounting Policies [Line Items]
Variable interest entity, qualitative or quantitative information, ownership percentage							30.00%
Ownership interest percentage increase on exercise of option			15.00%				70.00%
Assets								$ 105,191,000	102,456,000		$ 105,191,000	102,456,000
Liabilities								59,783,000	57,622,000		59,783,000	57,622,000
Doner | Aggregate 2017 Step Up Transactions [Domain]
Significant Accounting Policies [Line Items]
Ownership interest percentage increase on exercise of option			85.00%
Contingent Payment [Member]
Significant Accounting Policies [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability								$ 18,173,000	$ 9,211,000		$ 4,896,000	$ 7,972,000	36,344,000
Six Point Five Zero Percentage Notes [Domain]
Significant Accounting Policies [Line Items]
Debt Instrument, Interest Rate, Stated Percentage								6.50%			6.50%			6.50%
Six Point Seven Five Percentage Notes [Member]
Significant Accounting Policies [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						6.75%			6.75%			6.75%			6.75%	6.75%
Debt Instrument, Maturity Year						2020
Notes due 2024 [Domain]
Significant Accounting Policies [Line Items]
Debt Instrument, Interest Rate, Stated Percentage								6.50%			6.50%
Interest Expense [Member] | Fixed Payment [Member]
Significant Accounting Policies [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability											$ 100,000	$ 255,000	$ 2,543,000
Equity Method Investments [Member]
Significant Accounting Policies [Line Items]
Number of investments sold | investment												2
Proceeds from Sale of Equity Method Investments												$ 4,023
Equity Method Investment, Realized Gain (Loss) on Disposal												623
Cost-method Investments [Member]
Significant Accounting Policies [Line Items]
Step acquisition gain on sale of ownership percentage											3,018
Cost-method Investments, Realized Losses											$ 935
Cost Method Investments, Investments Dissolved											5
Cost-method Investments, Realized Gain (Loss)												$ 1,309
Number of investments sold | investment											3	3
Proceeds from sale of other investments											$ 3,557	$ 4,074
Quantitative Assessment [Member]
Significant Accounting Policies [Line Items]
Number of Reporting Units												13
Strategic Communications [Member]
Significant Accounting Policies [Line Items]
Reporting Units Impaired, Annual Goodwill Impairment Test												1
Goodwill, Impairment Loss									$ 18,893,000
Specialist Communications [Member]
Significant Accounting Policies [Line Items]
Goodwill, Impairment Loss											$ 0	$ 18,893,000
All Other [Domain]
Significant Accounting Policies [Line Items]
Reporting Units Impaired, Annual Goodwill Impairment Test											2
Goodwill, Impairment Loss											$ 3,238,000	29,631,000
Reporting Unit, Negative Carrying Value, Allocated Goodwill									$ 5,479,000			5,479,000
All Other [Domain] | Experiential [Member]
Significant Accounting Policies [Line Items]
Goodwill, Impairment Loss										$ 27,893,000
Reporting Units, Goodwill Impairment Test Performed During Interim Period | reportable_segment										1
All Other [Domain] | Immaterial [Member]
Significant Accounting Policies [Line Items]
Goodwill, Impairment Loss										$ 1,738,000
Reporting Units, Goodwill Impairment Test Performed During Interim Period | reportable_segment										1
Domestic Creative Agencies [Member]
Significant Accounting Policies [Line Items]
Goodwill, Impairment Loss											0	$ 0
Reporting Unit, Negative Carrying Value, Allocated Goodwill								$ 14,853,000			$ 14,853,000
Wells Fargo Capital Finance, Llc [Member]
Significant Accounting Policies [Line Items]
Debt Instrument, Maturity Date				May 03, 2021	Sep. 30, 2019
Wells Fargo Capital Finance, Llc [Member] | Revolving Credit Facility [Member]
Significant Accounting Policies [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 325,000,000	$ 0
Scenario, Forecast [Member] | Wells Fargo Capital Finance, Llc [Member] | Six Point Five Zero Percentage Notes [Domain]
Significant Accounting Policies [Line Items]
Debt Instrument, Maturity Year	2024