Acquisition of Bank of Granite Corporation (Merger Information) (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended
	Oct. 21, 2011	Mar. 31, 2012
Granite Corp. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Number of operating full-service branches	17
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
Additional goodwill recognized from the Merger		$ 0.3