Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 31, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Document Transition Report	false
Entity File Number	1-7265
Entity Registrant Name	AMBASE CORPORATION
Entity Central Index Key	0000020639
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	95-2962743
Entity Address, Address Line One	7857 WEST SAMPLE ROAD, SUITE 134
Entity Address, City or Town	CORAL SPRINGS
Entity Address, State or Province	FL
Entity Address, Postal Zip Code	33065
City Area Code	201
Local Phone Number	265-0169
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		40,737,751