| **Acquisition of Bank of Granite Corporation (Merger Information) (Details) (USD $)In Millions, except Share data, unless otherwise specified** | 0 Months Ended | 3 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| 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          |

| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Operating Full Service Branches
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares of equity interests issued or issuable to acquire entity.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of voting equity interests acquired at the acquisition date in the business combination.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face amount or stated value per share of common stock.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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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