Average holding size of bonds in the municipal bond portfolio.

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Number of pooled trust preferred securities carried by the entity.

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Number of single issuer trust preferred securities carried by the entity.

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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged to secure public deposits, trust deposits and for other purposes as required by law.

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Amortized cost of single issuer trust preferred securities carried by the entity.

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Fair value of single issuer trust preferred securities carried by the entity.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Bonds or similar securities issued by state, city, or local governments or the agencies operated by state, city, or local governments which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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-Glossary Cash Equivalents

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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-Publisher FASB

-Name Accounting Standards Codification

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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

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Securities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Security Securities	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Securities [Line Items]
Number of pooled issue securities	23fnb_NumberOfPooledIssueSecurities
Net proceeds from sale of securities	$ 175,872	$ 22,047	$ 87,101
Net securities gains (loss)	11,717us-gaap_GainLossOnSaleOfSecuritiesNet	808us-gaap_GainLossOnSaleOfSecuritiesNet	305us-gaap_GainLossOnSaleOfSecuritiesNet
Securities pledged to secure public deposits, trust deposits and for other purposes, carrying value	1,036,380	909,548
Securities pledged as collateral for short-term borrowings, carrying value	892,647	860,279
Number of single-issuer securities	4fnb_NumberOfSingleIssueSecurities
Estimated fair value of single-issue trust preferred securities	6,887
Amortized cost of single-issue trust preferred securities	6,030
Impairment losses on securities	0	27
Municipal bond portfolio, value	163,430
Municipal Debt Securities [Member] \| Combined A Rating [Member] \| Minimum [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	99.00%
Municipal Debt Securities [Member] \| General Obligation Bonds [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	99.60%
Municipal Debt Securities [Member] \| Pennsylvania, Ohio and Maryland [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	88.60%
Municipal Debt Securities [Member] \| Capital Support Agreement [Member]
Schedule Of Securities [Line Items]
Percentage of portfolio	86.00%
Municipal Debt Securities [Member] \| Weighted Average [Member]
Schedule Of Securities [Line Items]
Carrying amount per bond	1,143
Pooled TPS [Member]
Schedule Of Securities [Line Items]
Net proceeds from sale of securities	51,540
Net securities gains (loss)	13,766
Other Securities [Member]
Schedule Of Securities [Line Items]
Net securities gains (loss)	$ (2,351)