Document And Entity Information	3 Months Ended
	Mar. 31, 2012	May 15, 2012
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2012
Document Period End Date	Mar. 31, 2012
Current Fiscal Year End Date	--12-31
Entity Registrant Name	FNB United Corp.
Entity Central Index Key	0000764811
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		21,102,082