| **Investment Securities**                              | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              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| Investment Securities                                  | Investment SecuritiesSecurities designated as available-for-sale are carried at fair value.  However, the unrealized difference between amortized cost and fair value of securities available-for-sale is excluded from net income unless there is an other than temporary impairment and is reported, net of deferred taxes, as a component of shareholders' equity as accumulated other comprehensive income (loss).  Securities held-to-maturity are carried at amortized cost, as the banks have the ability, and management has the positive intent, to hold these securities to maturity.  Premiums and discounts on securities are amortized and accreted according to the interest method.The primary objective of FNB's management of the investment portfolio is to maintain a portfolio of high quality, highly liquid investments yielding competitive returns.  FNB is required under federal regulations to maintain adequate liquidity to ensure safe and sound operations.  FNB maintains investment balances based on a continuing assessment of cash flows, the level of loan production, current interest rate risk strategies and an assessment of the potential future direction of market interest rate changes.  Investment securities differ in terms of default, interest rate, liquidity and expected rate of return risks.The following table summarizes the amortized cost and estimated fair value of investment securities and presents the related gross unrealized gains and losses:March 31, 2013        (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair ValueAvailable for Sale:        Obligations of:        U.S. government sponsored enterprises $2,069 $38 $— $2,107Residential mortgage-backed securities-GSE 409,949 1,794 3,279 408,464Residential mortgage-backed securities-Private 21,992 879 250 22,621Commercial mortgage backed securities-GSE 23,054 — 658 22,396Corporate notes 36,567 240 40 36,767Total $493,631 $2,951 $4,227 $492,355Held to Maturity:        Residential mortgage-backed securities-GSE 73,515 1 271 73,245     Total $567,146 $2,952 $4,498 $565,600         December 31, 2012        (dollars in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair ValueAvailable for Sale:        Obligations of:        U.S. Treasury and government agencies $6,646 $335 $— $6,981U.S. government sponsored enterprises 22,118 55 — 22,173States and political subdivisions 5,918 120 — 6,038Residential mortgage-backed securities-GSE 436,344 5,678 948 441,074Residential mortgage-backed securities-Private 22,649 750 454 22,945Commercial mortgage backed securities-GSE 23,150 209 — 23,359Commercial mortgage backed securities-Private 5,283 34 — 5,317Corporate notes 36,710 270 17 36,963Total $558,818 $7,451 $1,419 $564,850CommunityOne and Granite, as members of the Federal Home Loan Bank of Atlanta (“FHLB”), are required to own capital stock in the FHLB based generally upon the balances of total assets and FHLB advances.  FHLB capital stock is pledged to secure FHLB advances.  This investment is carried at cost since no ready market exists for FHLB stock and there is no quoted market value.  However, redemption of this stock has historically been at par value.  The combined banks owned a total of $5.2 million of FHLB stock at March 31, 2013 and $6.3 million at December 31, 2012.  Due to the redemption provisions of FHLB stock, FNB estimated that fair value approximated cost and that this investment was not impaired at March 31, 2013.  FHLB stock is included in other assets at its original cost basis.CommunityOne, as a member bank of the Federal Reserve Bank of Richmond (“FRBR”), is required to own capital stock of the FRBR based upon a percentage of the bank's common stock and surplus.  This investment is carried at cost since no ready market exists for FRBR stock and there is no quoted market value.  At both March 31, 2013 and December 31, 2012, CommunityOne owned a total of $3.1 million, of FRBR stock.  Due to the nature of this investment in an entity of the U.S. government, FNB estimated that fair value approximated the cost and that this investment was not impaired at March 31, 2013.  FRBR stock is included in other assets at its original cost basis.At March 31, 2013, $102.7 million of the investment securities portfolio was pledged to secure public deposits, $13.9 million was pledged to retail repurchase agreements and $2.1 million was pledged to others, leaving $446.9 million available as lendable collateral.During the three months ended March 31, 2013 and 2012, the banks sold securities with a book value of  $145.9 million and $10.8 million respectively, and recognized a gain of $2.4 million and a loss of $(46) thousand, respectively. The banks sold these securities in order to manage our interest rate sensitivity profile.The following tables show our investments' estimated fair value and gross unrealized losses, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at March 31, 2013 and December 31, 2012.  All unrealized losses on investment securities are considered by management to be temporary given the credit ratings on these investment securities or the short duration of the unrealized loss or both. Less than 12 Months 12 Months or More Total(dollars in thousands)Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized Losses Estimated Fair ValueGross Unrealized LossesMarch 31, 2013        Available for Sale:        Residential mortgage-backed securities-GSE$284,396$3,279 $—$— $284,396$3,279Residential mortgage-backed securities-Private3,365217 4,02533 7,390250Commercial mortgage-backed securities-GSE22,396658 —— 22,396658Corporate notes—— 3,23440 3,23440Total$310,157$4,154 $7,259$73 $317,416$4,227Held to Maturity:        Residential mortgage-backed securities-GSE$48,036$272 $—$— $48,036$272         December 31, 2012        Obligations of:        Residential mortgage-backed securities-GSE$123,489$904 $7,027$44 $130,516$948Residential mortgage-backed securities-Private6,482438 1,01716 7,499454Corporate notes—— 3,24917 3,24917Total$129,971$1,342 $11,293$77 $141,264$1,419At March 31, 2013, there were four available-for-sale securities that were in an unrealized loss position for longer than 12 months, compared to four at December 31, 2012.If an entity intends to sell a debt security or cannot assert it is more likely than not that it will not have to sell the security before recovery, other than temporary impairment ("OTTI") must be taken.  If the entity does not intend to sell the debt security before recovery, but the entity does not expect to recover the entire amortized cost basis, then OTTI must be taken, but the amount of impairment is to be bifurcated between impairment due to credit (which is recorded through earnings) and noncredit impairment (which becomes a component of other comprehensive income (“OCI”) for both available-for-sale and held-to-maturity securities).  For held-to-maturity securities, the amount in OCI will be amortized prospectively over the security's remaining life. After analyzing its securities portfolio at March 31, 2013, and after considering ratings, fair value, cash flows and other factors,  FNB did not have any OTTI during the three months ended March 31, 2013 and March 31, 2012.The aggregate amortized cost and fair value of securities at March 31, 2013, by remaining contractual maturity, are shown in the following table.  Actual expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations.  Available-for-Sale Held-to-Maturity(dollars in thousands) Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair ValueDue in one year or less $16,458 $16,551 $— $—Due after one year through five years 18,904 19,089 — —Due after five years through 10 years 3,274 3,234 — —Due after 10 years — — — —Total 38,636 38,874 — —Mortgage-backed securities 454,995 453,481 73,515 73,245Total $493,631 $492,355 $73,515 $73,245 |

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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27405-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 3, 19, 20, 21, 22, 137 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Equity Security -URI http://asc.fasb.org/extlink&oid=6511694Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Debt Security -URI http://asc.fasb.org/extlink&oid=6509901Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27232-111563Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563
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