Commitments, Credit Risk and Contingencies - Additional Information (Detail)	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Percentage of commitments to extend credit dependent upon the financial condition of the customers	74.20%

Percentage of commitments to extend credit that are dependent of the financial condition of the customer.

No definition available.

Name:	fnb_PercentageOfCommitmentsToExtendCreditDependentOnFinancialCondition
Namespace Prefix:	fnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant