| **Acquisition of Bank of Granite Corporation (Merger Information) (Details) (USD $)** |                 |                 |               | 0 Months Ended | 3 Months Ended |
|---------------------------------------------------------------------------------------|-----------------|-----------------|---------------|----------------|----------------|
||
| **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      |

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Number of Operating Full Service Branches
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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| - Definition
Number of shares of equity interests issued or issuable to acquire entity.
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| - Definition
Percentage of voting equity interests acquired in the business combination.
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| - Definition
Face amount or stated value of common stock per share; generally not indicative of the fair market value per share.
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| - Definition
Purchase accounting adjustments needed to revise the carrying amount of goodwill to fair value.
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