Long-Term Borrowings - Junior Subordinated Debt Trusts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Subordinated Borrowing [Line Items]
Junior subordinated debt	$ 66	$ 111
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
Stated Maturity Date	Jun. 15, 2036
Interest Rate	3.54%
Description of variable rate	LIBOR + 165 basis points (bps)
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
Stated Maturity Date	Dec. 15, 2037
Interest Rate	3.21%
Description of variable rate	LIBOR + 132 bps
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	$ 22
Stated Maturity Date	Sep. 30, 2035
Interest Rate	3.42%
Description of variable rate	LIBOR + 146 bps
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