Acquisition of Bank of Granite Corporation (Merger Information) (Details) (USD $)				0 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Granite Corp. [Member] banks employee	Oct. 21, 2011 Granite Corp. [Member]	Mar. 31, 2012 Granite Corp. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Stock exchange transaction				100.00%
Common stock, par value (in usd per share)				$ 1.00
Shares of FNB common stock each outstanding acquiree share is convertible into				3.375
FNB common stock issued upon Merger				521,595
Number of operating full-service branches			17
Total assets	$ 2,151,565,000	$ 2,409,108,000	$ 682,300,000
Number of employees			148
Additional goodwill recognized from the Merger					$ 300,000