Acquisitions - Textual (Details)				12 Months Ended
	Jul. 01, 2016 USD ($) shares	Jul. 01, 2016 CAD shares	Mar. 28, 2012	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($)	Apr. 01, 2016	Oct. 31, 2015	May 01, 2015	Aug. 18, 2014	Aug. 01, 2014	Jul. 31, 2014	Jun. 03, 2014	Feb. 14, 2014	Jan. 01, 2014
Business Acquisition [Line Items]
Pro forma revenue				$ 1,426,770,000	$ 1,398,756,000
Pro forma net income (loss)				(39,756,000)	(34,398,000)
Goodwill				$ 24,778,000	43,654,000
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests						$ 17,831,000
Intangible assets amortization period (years)				7 years
Ownership percentage by parent				100.00%
Adjustments to deferred acquisition consideration included in share-based compensation				$ 10,341	$ 9,359	7,802
Stock Issued During Period, Value, Acquisitions				$ 34,219,000
Basic and Diluted Earnings Per Share, Pro Forma | $ / shares				$ (0.76)	$ (0.66)
Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Cost of acquired entity, cash paid				$ 1,581
Business Combinations, Assets Relinquished by Acquirer				428
Business Combinations, Liabilities Relinquished Previously due to Acquiree				94
Aggregate 2015 Acquisitions [Domain]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred					$ 55,279
Cost of acquired entity, cash paid					23,000
Business Combination, Consideration Transferred, Liabilities Incurred					32,279
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					16,721
Goodwill					$ 43,654
Intangible assets amortization period (years)					6 years 4 months
Business Acquisition, Goodwill, Expected Tax Deductible Amount					$ 9,720
Aggregate 2015 Step Up Transactions [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred					200,822
Cost of acquired entity, cash paid					37,467
Business Combination, Consideration Transferred, Liabilities Incurred					163,355
Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Business Combination, Consideration Transferred						151,202,000
Cost of acquired entity, cash paid						67,236,000
Business Combination, Consideration Transferred, Liabilities Incurred						83,966,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles						64,733,000
Goodwill						146,806,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount						$ 149,232,000
Aggregate 2014 Acquisitions | Minimum
Business Acquisition [Line Items]
Intangible assets amortization period (years)						5 years
Aggregate 2014 Acquisitions | Maximum
Business Acquisition [Line Items]
Intangible assets amortization period (years)						6 years
Trapeze Media Limited | Union Advertising Canada LC
Business Acquisition [Line Items]
Step acquisition gain on sale of ownership percentage						$ 908,000
Forsman & Bodenfors AB [Domain]
Business Acquisition [Line Items]
Pro forma revenue				39,569,000
Pro forma net income (loss)				3,815,000
Percentage of voting interests acquired	100.00%
Business Combination, Consideration Transferred	$ 49,837
Business Combination, Consideration Transferred, Liabilities Incurred				18,857
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	36,698
Goodwill	$ 24,778
Intangible assets amortization period (years)	10 years 9 months	10 years 9 months
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	$ 5,514
Business Acquisition, Goodwill, Expected Tax Deductible Amount	0
Deferred Taxes, Business Combination, Valuation Allowance, Available to Reduce Intangible Assets	8,074
Business Combination, Contingent Consideration, Liability	15,618
Forsman & Bodenfors AB [Domain] | Minimum
Business Acquisition [Line Items]
Business Combination, Consideration Transferred | CAD		CAD 35,000
Forsman & Bodenfors AB [Domain] | Maximum
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	$ 55,000
Y Media Labs LLC [Domain] | Aggregate 2015 Acquisitions [Domain]
Business Acquisition [Line Items]
Percentage of voting interests acquired									60.00%
Unique Influence LLC [Domain] | Aggregate 2015 Acquisitions [Domain]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Net Assets Acquired								100.00%
Luntz Global Partners LLC
Business Acquisition [Line Items]
Percentage of voting interests acquired							40.00%
Luntz Global Partners LLC | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Percentage of voting interests acquired															60.00%
Kingsdale Partners, LP | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Percentage of voting interests acquired														65.00%
The House Worldwide Ltd | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Percentage of voting interests acquired													100.00%
Hunter PR LLC | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Percentage of voting interests acquired											65.00%
Albion Brand Communications Limited | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Percentage of voting interests acquired										75.00%
Trapeze Media Limited
Business Acquisition [Line Items]
Ownership percentage by parent												18.00%
Doner
Business Acquisition [Line Items]
Percentage of voting interests acquired			70.00%
Variable interest entity, qualitative or quantitative information, ownership percentage			30.00%
Additional Paid-in Capital
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests						8,992,000
Additional Paid-in Capital | Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests				22,775
Additional Paid-in Capital | Aggregate 2015 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests					42,780
Noncontrolling Interest [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests						8,839,000
Noncontrolling Interest [Member] | Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests				19,354
Noncontrolling Interest [Member] | Aggregate 2015 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests					8,708
Noncontrolling Interest [Member] | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value						50,552,000
Redeemable Noncontrolling Interest [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests				1,708,000	155,042,000	4,820,000
Redeemable Noncontrolling Interest [Member] | Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests				1,005
Redeemable Noncontrolling Interest [Member] | Aggregate 2015 Acquisitions [Domain]
Business Acquisition [Line Items]
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value					1,999
Redeemable Noncontrolling Interest [Member] | Aggregate 2015 Step Up Transactions [Member]
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests					149,335
Redeemable Noncontrolling Interest [Member] | Aggregate 2014 Acquisitions
Business Acquisition [Line Items]
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value						13,327,000
Contingent payment [Domain] | Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred				4,052
Business Combination, Deferred Stock-based Compensation, Discounted				2,393
Fixed payments [Member] | Aggregate 2016 Step Up Transactions [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred				$ 467
Common Class A | Common Stock [Member]
Business Acquisition [Line Items]
Stock Issued During Period, Shares, Acquisitions | shares				1,900,000
Common Class A | Common Stock [Member] | Forsman & Bodenfors AB [Domain]
Business Acquisition [Line Items]
Stock Issued During Period, Shares, Acquisitions | shares	1,900,000	1,900,000
Stock Issued During Period, Value, Acquisitions	$ 34,219			$ 34,219,000	$ 0	$ 0
Contractual redemption right [Member] | Forsman & Bodenfors AB [Domain]
Business Acquisition [Line Items]
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	$ 2,275