| **Basis of Presentation**                                                           | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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| **Organization, Consolidation and Presentation of Financial Statements [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           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| Basis of Presentation                                                               | Basis of PresentationNature of OperationsFNB United Corp., we or us (which also refers to FNB and our subsidiaries on a consolidated basis) ("FNB"), is a bank holding company incorporated in 1984 under the laws of the State of North Carolina. We own two bank subsidiaries: CommunityOne Bank, N.A. (“CommunityOne”), a national banking association headquartered in Asheboro, North Carolina and, through Bank of Granite Corporation (“Granite Corp.”), Bank of Granite (“Granite”), a state chartered bank headquartered in Granite Falls, North Carolina. On March 1, 2013, we filed an application to merge Granite into CommunityOne. Subject to regulatory approval, we expect to complete the Granite Merger in the second quarter of 2013.   Through our bank subsidiaries, we offer a complete line of consumer, wealth management, mortgage and business banking services, including loan, deposit, cash management, investment management and trust services, to individual and business customers through operations located throughout central, southern and western North Carolina, including the counties of Alamance, Alexander, Ashe, Burke, Caldwell, Catawba, Chatham, Gaston, Guilford, Iredell, Mecklenburg, Montgomery, Moore, Orange, Randolph, Richmond, Rowan, Scotland, Watauga and Wilkes.CommunityOne owns two subsidiaries:  Dover Mortgage Company (“Dover”) and First National Investor Services, Inc. Dover previously engaged in the business of originating, underwriting and closing mortgage loans for sale in the secondary market.  Dover ceased operations in the first quarter of 2011 and filed for Chapter 11 bankruptcy on February 15, 2012.  First National Investor Services, Inc. holds deeds of trust for CommunityOne.  Through Granite Corp., we also own Granite Mortgage, Inc., which ceased mortgage operations in 2009 and filed for Chapter 11 bankruptcy on February 15, 2012.  FNB also owns FNB United Statutory Trust I, FNB United Statutory Trust II, and Catawba Valley Capital Trust II, which were formed to facilitate the issuance of trust preferred securities.On October 21, 2011, as part of the recapitalization of FNB, FNB acquired Granite Corp., through the merger of a wholly owned subsidiary of FNB merging into Granite Corp. (the "Merger").  The Merger was part of FNB's recapitalization strategy.GeneralThe accompanying consolidated financial statements, prepared without audit, include the accounts of FNB and its subsidiaries.  All significant intercompany balances and transactions have been eliminated.  Descriptions of the organization and business of FNB, accounting policies followed by FNB and other relevant information are contained in FNB's Annual Report on Form 10-K for the year ended December 31, 2012, (the "Form 10-K"), including the notes to the consolidated financial statements filed as part of that report.  This quarterly report should be read in conjunction with the Form 10-K.In the opinion of management, the accompanying condensed consolidated financial statements contain all the adjustments, all of which are normal recurring adjustments, necessary to present fairly the financial position of FNB as of March 31, 2013 and December 31, 2012, and the results of its operations and cash flows for the three months ended March 31, 2013 and 2012, respectively.All financial information is reported on a continuing operations basis, unless otherwise noted.  See Note 2 to the consolidated financial statements for a discussion regarding discontinued operations and certain assets and liabilities at March 31, 2013 and December 31, 2012.Use of EstimatesWe have made a number of estimates and assumptions relating to the reporting of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period to prepare these consolidated financial statements in conformity with accounting principles generally accepted in the United States ("GAAP").  Actual results could differ from those estimates.  Material estimates subject to change in the near term include, among other items, the allowances for loan losses (“ALL”), the carrying value of other real estate owned (“OREO”), the carrying value of intangible assets and the realization of deferred tax assets.ReclassificationCertain reclassifications have been made to the prior period consolidated financial statements to place them on a comparable basis with the current period consolidated financial statements.  These reclassifications have no effect on net income or shareholders' equity as previously reported.Subsequent EventsSubsequent events are events or transactions that occur after the balance sheet date but before financial statements are issued.  Recognized subsequent events are events or transactions that provide additional evidence about conditions that existed at the date of the balance sheet, including the estimates inherent in the process of preparing financial statements.  Non-recognized subsequent events are events that provide evidence about conditions that did not exist at the date of the balance sheet but arose after that date.  Management has reviewed events occurring through the date of this filing and has concluded that no subsequent events have occurred requiring accrual or disclosure in addition to that included herein.Recent Accounting PronouncementsComprehensive Income - In February 2013, the FASB issued ASU No. 2013-02 Comprehensive Income (Topic 220): Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income (“ASU No. 2013-02”). This pronouncement requires an entity to report the effect of significant reclassifications out of accumulated other comprehensive income for each applicable component of net income, as well as a rollforward of the components of accumulated other comprehensive income on a prospective basis. This pronouncement is effective beginning January 1, 2013. The provisions of ASU No. 2013-02 relate only to financial statement presentation of other comprehensive income and, accordingly, its adoption did not have a material effect on FNB's financial statements. See Note 10 for further disclosure.FASB - From time to time, the Financial Accountings Standards Board (“FASB”) issues exposure drafts for proposed statements of financial accounting standards. Such exposure drafts are subject to comment from the public, to revisions by the FASB and to final issuance by the FASB as statements of financial accounting standards. Management considers the effect of the proposed statements on the consolidated financial statements of FNB and monitors the status of changes to and proposed effective dates of exposure drafts. Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies are not expected to have a material impact on FNB’s financial position, results of operations or cash flows. |

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The entire disclosure for organization, consolidation and basis of presentation of financial statements disclosure.
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