Acquisition of Bank of Granite Corporation (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

Business Acquisition, Equity Interest Issued or Issuable, Conversion Ratio

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- Definition

Number of Operating Full Service Branches

+ References

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- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

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- Definition

Face amount or stated value per share of common stock.

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-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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