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STOCK INCENTIVE PLANS - Schedule of Issuance of Restricted Stock Units and Aggregate Weighted Average Grant Date Fair Values (Detail) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018
Share-based Payment Arrangement [Abstract] |  |
Restricted stock units issued (in shares) | 1,128,701 | 937,155
Weighted average grant date fair values | $ 12 | $ 12

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- Definition The weighted average fair value amount of nonvested equity-based awards other than stock option plans on the grant date. + References No definition available. + Details
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Namespace Prefix: | fnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration