Consolidated Statements of Convertible Preferred Stock, Redeemable Noncontrolling Interest and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total stockholders' (Deficit) equity attributable to Millendo Therapeutics, Inc.	Common Stock Common-1 stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Total Equity Attributable to Noncontrolling Interests	Convertible preferred stock	Redeemable Noncontrolling Interests	Total
Convertible Preferred Stock, balance at beginning at Dec. 31, 2017								$ 132,922	$ 10,584
Convertible Preferred Stock, balance at beginning (in shares) at Dec. 31, 2017								90,848,515
Convertible Preferred Stock
Foreign currency translation adjustment									10
Net income (loss)									(676)
Convertible Preferred Stock, balance at end at Sep. 30, 2018								$ 132,922	9,918
Convertible Preferred Stock, balance at end (in shares) at Sep. 30, 2018								90,848,515
Balance at beginning at Dec. 31, 2017	$ (130,694)			$ 6,192	$ (136,894)	$ 8	$ 2,171			$ (128,523)
Balance at beginning (in shares) at Dec. 31, 2017		464,043	246,347
Stockholder's equity
Stock-based compensation expense	759			759						759
Foreign currency translation adjustment	48					48				48
Net income (loss)	(16,643)				(16,643)					(16,643)
Balance at end at Sep. 30, 2018	(146,530)			6,951	(153,537)	56	2,171			(144,359)
Balance at end (in shares) at Sep. 30, 2018		464,043	246,347
Convertible Preferred Stock, balance at beginning at Jun. 30, 2018								$ 132,922	10,270
Convertible Preferred Stock, balance at beginning (in shares) at Jun. 30, 2018								90,848,515
Convertible Preferred Stock
Foreign currency translation adjustment									3
Net income (loss)									(355)
Convertible Preferred Stock, balance at end at Sep. 30, 2018								$ 132,922	$ 9,918
Convertible Preferred Stock, balance at end (in shares) at Sep. 30, 2018								90,848,515
Balance at beginning at Jun. 30, 2018	(139,425)			6,528	(146,001)	48	2,171			(137,254)
Balance at beginning (in shares) at Jun. 30, 2018		464,043	246,347
Stockholder's equity
Stock-based compensation expense	423			423						423
Foreign currency translation adjustment	8					8				8
Net income (loss)	(7,536)				(7,536)					(7,536)
Balance at end at Sep. 30, 2018	(146,530)			6,951	(153,537)	56	2,171			(144,359)
Balance at end (in shares) at Sep. 30, 2018		464,043	246,347
Balance at beginning at Dec. 31, 2018	70,951		$ 13	234,876	(164,086)	148	2,171			73,122
Balance at beginning (in shares) at Dec. 31, 2018			13,357,999
Stockholder's equity
Exercise of stock options	360			360						$ 360
Exercise of stock options (in shares)			97,225							97,225
Exercise/forfeiture of BSPCE warrants	527			527			(416)			$ 111
Exercise/forfeiture of BSPCE warrants (in shares)			17,713
Stock-based compensation expense	3,130			3,130						3,130
Foreign currency translation adjustment	(25)					(25)				(25)
Net income (loss)	(31,869)				(31,869)					(31,869)
Balance at end at Sep. 30, 2019	43,074		$ 13	238,893	(195,955)	123	1,755			44,829
Balance at end (in shares) at Sep. 30, 2019			13,472,937
Balance at beginning at Jun. 30, 2019	52,966		$ 13	237,136	(184,323)	140	2,059			55,025
Balance at beginning (in shares) at Jun. 30, 2019			13,412,058
Stockholder's equity
Exercise of stock options	194			194						194
Exercise of stock options (in shares)			51,278
Exercise/forfeiture of BSPCE warrants	367			367			(304)			63
Exercise/forfeiture of BSPCE warrants (in shares)			9,601
Stock-based compensation expense	1,196			1,196						1,196
Foreign currency translation adjustment	(17)					(17)				(17)
Net income (loss)	(11,632)				(11,632)					(11,632)
Balance at end at Sep. 30, 2019	$ 43,074		$ 13	$ 238,893	$ (195,955)	$ 123	$ 1,755			$ 44,829
Balance at end (in shares) at Sep. 30, 2019			13,472,937