RETIREMENT PLANS - Schedule of Fair Value of Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 170	$ 201	$ 189
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	7
F.N.B. Corporation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	4
Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	3
Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	67
Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	6
Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	17
U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	14
U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	44
U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	23
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	130	157
Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	7
Level 1 | F.N.B. Corporation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	4
Level 1 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	3
Level 1 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	67
Level 1 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Level 1 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
Level 1 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	6
Level 1 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	17
Level 1 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	14
Level 1 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
Level 1 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	5
Level 1 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	23
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	44
Level 2 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | F.N.B. Corporation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	44
Level 2 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | F.N.B. Corporation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0