Acquisitions and Dispositions (Details Textual) - USD ($)					3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 07, 2018	Jul. 01, 2018	Apr. 02, 2018	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Redeemable Noncontrolling Interest, Equity, Fair Value					$ 57,193,000			$ 57,193,000		$ 62,886,000	$ 60,180,000
Noncontrolling interest, ownership percentage by Parent (percent)					100.00%			100.00%
Adjustments to deferred acquisition consideration included in share-based compensation					$ 3,076,000	$ 3,160,000		$ 7,758,000	$ 7,080,000
Goodwill					843,180,000			843,180,000		835,935,000
Deferred acquisition consideration					94,729,000			94,729,000		122,426,000
Business acquisitions and step-up transactions								19,219,000
Noncontrolling Interest, Increase from Business Combination								0		1,666,000
Stock-based compensation					6,242,000	6,380,000		16,882,000	16,870,000
Net loss attributable to MDC Partners Inc.					(16,125,000)	18,493,000		(42,135,000)	19,180,000
Change from net loss attributable to MDC Partners Inc. and transfers to noncontrolling interests					(11,150,000)	18,156,000		(38,326,000)	21,495,000
Changes in redemption value								(5,965,000)
Business Combination, Acquisition Related Costs					232,000	216,000		943,000	693,000
Aggregate 2018 Step-up Transaction [Member] [Member]
Business Acquisition [Line Items]
Closing cash payment			$ 3,279,000
Allison & Partners LLC
Business Acquisition [Line Items]
Remaining ownership interest acquired (percent)				24.50%
Aggregate purchase price				$ 10,023,000
Closing cash payment				300,000
Deferred acquisition consideration				9,723,000
Business acquisitions and step-up transactions				(8,857,000)
Noncontrolling Interest, Increase from Business Combination				$ 1,166,000
Aggregate 2017 Step-up Transaction
Business Acquisition [Line Items]
Closing cash payment							$ 3,352,000
Deferred acquisition consideration							7,208,000
Business acquisitions and step-up transactions							(11,947,000)
Noncontrolling Interest, Increase from Business Combination							2,652,000
Stock-based compensation							997,000
Reduction in noncontrolling interests							$ 269,000
Aggregate 2017 Dispositions [Member]
Business Acquisition [Line Items]
Loss on disposition of business										1,424,000
Goodwill, Written off Related to Sale of Business Unit										17,593
Noncontrolling Interest, Decrease from Deconsolidation										$ 10,657
Parent [Member]
Business Acquisition [Line Items]
Net loss attributable to MDC Partners Inc.								$ (42,135,000)
Common Stock [Member]
Business Acquisition [Line Items]
Stock Issued During Period, Shares, Acquisitions								1,011,561
Noncontrolling Interest
Business Acquisition [Line Items]
Changes in redemption value								$ (5,965,000)
OneChocolate [Domain]
Business Acquisition [Line Items]
Remaining ownership interest acquired (percent)	100.00%
Aggregate purchase price	$ 3,231,000
Doner [Domain]
Business Acquisition [Line Items]
Remaining ownership interest acquired (percent)		14.90%
Source [Domain]
Business Acquisition [Line Items]
Remaining ownership interest acquired (percent)		3.00%
Doner and Source [Domain]
Business Acquisition [Line Items]
Aggregate purchase price		$ 7,618,000
Business acquisitions and step-up transactions		(11,946,000)
Reduction in noncontrolling interests		$ 933,000
Instrument [Domain]
Business Acquisition [Line Items]
Remaining ownership interest acquired (percent)			51.00%
Business Combination, Consideration Transferred, Other			$ 10,304,000
Finite-lived Intangible Assets Acquired			23,130,000
Goodwill			29,514,000
Aggregate purchase price			35,591,000
Closing cash payment			28,561,000
Noncontrolling Interest, Increase from Business Combination			$ 27,357,000
Common Class A [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Stock Issued During Period, Value, Acquisitions								7,030,000	28,727,000
Common Class A [Member] | Instrument [Domain] | Common Stock [Member]
Business Acquisition [Line Items]
Stock Issued During Period, Shares, Acquisitions			1,011,561
Stock Issued During Period, Value, Acquisitions			$ 7,030,000
Contingent Payment [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability					$ 11,003,000	$ (2,462,000)		$ 8,522,000	$ 12,152,000