UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' DEFICIT - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	MDC Partners Inc. Shareholders' Deficit	Noncontrolling Interest	Series 4 Convertible Preferred Stock [Member] Convertible Preferred Stock
Balance at Dec. 31, 2018	$ (257,590)	$ 90,123		$ 58,579	$ (475,526)	$ 4,720	$ (322,104)	$ 64,514
Balance (in shares) at Dec. 31, 2018									95,000,000
Common stock, balance (in shares) at Dec. 31, 2018			57,521,323
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss attributable to MDC Partners Inc.	2,425				2,425		2,425
Other comprehensive income	(7,505)					(7,598)	(7,598)	93
Issuance of common and preference shares (in shares)		50,000,000	14,285,714
Issuance of common and convertible preference shares	98,620	$ 62,623		35,997			98,620
Issuance of restricted stock	0
Issuance of restricted stock (in shares)			566,932
Shares acquired and cancelled	(577)			(577)			(577)
Shares acquired and cancelled (in shares)			(227,552)
Stock-based compensation	1,918			1,918			1,918
Changes in redemption value of redeemable noncontrolling interests	3,496			3,496			3,496
Business acquisitions and step-up transactions, net of tax	(745)			(745)			(745)
Changes in ownership interest	(24,935)			(293)			(293)	(24,642)
Business acquisitions and step-up transactions, net of tax							745
Other	25			11	5		16	9
Balance at Sep. 30, 2019	(184,918)	152,746		98,364	(473,106)	(2,878)	(224,874)	39,956
Balance (in shares) at Sep. 30, 2019									145,000,000
Common stock, balance (in shares) at Sep. 30, 2019			72,146,417
Balance at Jun. 30, 2019	(182,600)	152,746		97,455	(471,349)	(1,713)	(222,861)	40,261
Balance (in shares) at Jun. 30, 2019									145,000,000
Common stock, balance (in shares) at Jun. 30, 2019			71,947,743
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss attributable to MDC Partners Inc.	(1,752)				(1,752)		(1,752)
Other comprehensive income	(1,461)					(1,165)	(1,165)	(296)
Issuance of restricted stock	0
Issuance of restricted stock (in shares)			372,953
Shares acquired and cancelled	(499)			(499)			(499)
Shares acquired and cancelled (in shares)			(174,279)
Stock-based compensation	1,409			1,409			1,409
Changes in redemption value of redeemable noncontrolling interests	767			767			767
Business acquisitions and step-up transactions, net of tax	(648)			(648)			(648)
Changes in ownership interest	(109)			(109)			(109)	0
Business acquisitions and step-up transactions, net of tax							(648)
Other	25			11	5		16	9
Balance at Sep. 30, 2019	(184,918)	152,746		98,364	(473,106)	(2,878)	(224,874)	39,956
Balance (in shares) at Sep. 30, 2019									145,000,000
Common stock, balance (in shares) at Sep. 30, 2019			72,146,417
Balance at Dec. 31, 2019	(190,575)	152,746		101,469	(480,779)	(4,269)	(230,833)	40,258
Balance (in shares) at Dec. 31, 2019									145,000,000
Common stock, balance (in shares) at Dec. 31, 2019			72,154,603
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss attributable to MDC Partners Inc.	4,486				4,486		4,486
Other comprehensive income	11,171					11,036	11,036	135
Issuance of restricted stock	0
Issuance of restricted stock (in shares)			1,807,984
Shares acquired and cancelled	(861)			(861)			(861)
Shares acquired and cancelled (in shares)			(407,894)
Stock-based compensation	5,243			5,243			5,243
Changes in redemption value of redeemable noncontrolling interests	(1,056)			(1,056)			(1,056)
Business acquisitions and step-up transactions, net of tax	1,626			1,626			(1,626)
Other	(383)			(384)	0	1	(383)	0
Balance at Sep. 30, 2020	(170,349)	152,746		106,037	(476,293)	6,768	(210,742)	40,393
Balance (in shares) at Sep. 30, 2020									145,000,000
Common stock, balance (in shares) at Sep. 30, 2020			73,554,693
Balance at Jun. 30, 2020	(184,603)	152,746		98,234	(480,368)	4,627	(224,761)	40,158
Balance (in shares) at Jun. 30, 2020									145,000,000
Common stock, balance (in shares) at Jun. 30, 2020			72,587,390
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss attributable to MDC Partners Inc.	4,076						4,076
Other comprehensive income	2,375					2,141	2,141	234
Issuance of restricted stock	0
Issuance of restricted stock (in shares)			1,047,423
Shares acquired and cancelled	(129)			(129)			(129)
Shares acquired and cancelled (in shares)			(80,120)
Stock-based compensation	4,210			4,210			4,210
Changes in redemption value of redeemable noncontrolling interests	1,574			1,574			1,574
Business acquisitions and step-up transactions, net of tax	2,129			2,129			2,129
Other	19			19	(1)		18	1
Balance at Sep. 30, 2020	$ (170,349)	$ 152,746		$ 106,037	$ (476,293)	$ 6,768	$ (210,742)	$ 40,393
Balance (in shares) at Sep. 30, 2020									145,000,000
Common stock, balance (in shares) at Sep. 30, 2020			73,554,693