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Securities (Summary Of Cumulative Credit-Related OTTI Charges) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2011 | Mar. 31, 2012
Securities [Abstract] |  |
Beginning balance of the amount related to credit loss for which a portion of OTTI was recognized in other comprehensive income | $ (18,332) | $ (18,398)
Additions related to credit loss for securities with previously recognized OTTI | (37) |
Additions related to credit loss for securities with initial OTTI | (29) |
Ending balance of the amount related to credit loss for which a portion of OTTI was recognized in other comprehensive income | $ (18,398) | $ (18,398)

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- Definition Additions related to credit loss for securities with initial other than temporary impairment. + References No definition available. + Details
Name: | fnb_AdditionsRelatedToCreditLossForSecuritiesWithInitialOtherThanTemporaryImpairment
Namespace Prefix: | fnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Additions related to credit loss for securities with previously recognized other than temporary impairment. + References No definition available. + Details
Name: | fnb_AdditionsRelatedToCreditLossForSecuritiesWithPreviouslyRecognizedOtherThanTemporaryImpairment
Namespace Prefix: | fnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Credit loss portion of other than temporary impairment recognized from Other comprehensive income. + References No definition available. + Details
Name: | fnb_CreditLossPortionOfOtherThanTemporaryImpairmentRecognizedFromOtherComprehensiveIncome
Namespace Prefix: | fnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_MarketableSecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration