Long-Term Borrowings - Junior Subordinated Debt Trusts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 67	$ 66
Trust Preferred Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	72
Common Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	3
F.N.B. Statutory Trust II
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 22
Interest Rate	1.85%
Basis points	1.65%
F.N.B. Statutory Trust II | Trust Preferred Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 22
F.N.B. Statutory Trust II | Common Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	1
Yadkin Valley Statutory Trust I
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 22
Interest Rate	1.52%
Basis points	1.32%
Yadkin Valley Statutory Trust I | Trust Preferred Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 25
Yadkin Valley Statutory Trust I | Common Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	1
FNB Financial Services Capital Trust I
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 23
Interest Rate	1.68%
Basis points	1.46%
FNB Financial Services Capital Trust I | Trust Preferred Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 25
FNB Financial Services Capital Trust I | Common Securities
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 1