Document And Entity Information	9 Months Ended
	Sep. 30, 2012	Nov. 01, 2012
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2012
Document Period End Date	Sep. 30, 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		21,588,056