UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' DEFICIT - 9 months ended Sep. 30, 2018 - USD ($) $ in Thousands	Total	Convertible Preferred Stock Series 4 Convertible Preferred Stock	Common Stock	Common Stock Common Class A [Member]	Additional Paid-in Capital	Charges in Excess of Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	MDC Partners Inc. Shareholders' Deficit	Noncontrolling Interest	Contingent Consideration, Liability Settlements [Domain]	Contingent Consideration, Liability Settlements [Domain] MDC Partners Inc. Shareholders' Deficit
Stock Issued During Period, Shares, Acquisitions			1,011,561
Balance at Dec. 31, 2017	$ (155,513)	$ 90,220	$ 352,432		$ 0	$ (314,241)	$ (340,000)	$ (1,954)	$ (213,543)	$ 58,030
Balance (in shares) at Dec. 31, 2017		95,000	56,375,131	56,371,376
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss attributable to MDC Partners Inc.	(42,135)								(42,135)
Other comprehensive income (loss)	(898)							635	635	(1,533)
Issuance of Series 4 convertible preference shares in private placement (in shares)		0
Expenses for convertible preference shares (Note 9)	(97)	$ (97)							(97)
Issuance of restricted stock	0		$ 3,509		(3,509)
Issuance of restricted stock (in shares)			237,529
Shares acquired and cancelled	(776)		$ (776)						(776)
Shares acquired and cancelled (in shares)			(108,782)
Stock-based compensation	6,774				6,774				6,774
Changes in redemption value of redeemable noncontrolling interests	(4,409)				(4,409)				(4,409)
Adjustments to Additional Paid in Capital, Changes due to Business Combinations					(3,809)				(3,809)	15,410
Business acquisitions and step-up transactions	19,219
Changes in ownership interest	(5,965)									(5,965)
Cumulative effect of adoption of ASC 606 (Note 14)	(1,170)						(1,170)		(1,170)
Transfer to charges in excess of capital	0				(2,665)	2,665
Balance at Sep. 30, 2018	$ (177,940)	$ 90,123	$ 362,195		$ 0	$ (311,576)	$ (383,305)	$ (1,319)	$ (243,882)	$ 65,942
Balance (in shares) at Sep. 30, 2018		95,000	57,515,439	57,511,684
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, Acquisitions				$ 7,030							$ 7,030	$ 7,030