Business Segments - Financial Information for Segments of FNB (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest income	$ 270	$ 273	$ 281	$ 306	$ 306	$ 314	$ 317	$ 310	$ 1,130	$ 1,247	$ 1,170
Interest expense	36	46	52	74	80	84	87	79	208	330	238
Net Interest Income	234	227	229	232	226	230	230	231	922	917	932
Provision for credit losses	18	27	30	48	7	12	11	14	123	44	61
Non-interest income	68	80	77	69	74	80	75	65	294	294	276
Non-interest expense									737	682	679
Amortization of intangibles									13	14	16
Income tax expense (benefit)									57	84	79
Net income (loss)	72	$ 83	$ 84	$ 47	95	$ 103	$ 95	$ 94	286	387	373
Total assets	37,354				34,615				37,354	34,615	33,102
Total intangibles	2,316				2,329				2,316	2,329	2,334
Operating Segments | Community Banking
Segment Reporting Information [Line Items]
Interest income									1,129	1,245	1,145
Interest expense									188	310	219
Net Interest Income									941	935	926
Provision for credit losses									123	44	54
Non-interest income									233	237	213
Non-interest expense									675	621	609
Amortization of intangibles									12	13	15
Income tax expense (benefit)									61	88	82
Net income (loss)									303	406	379
Total assets	37,245				34,491				37,245	34,491	32,997
Total intangibles	2,279				2,291				2,279	2,291	2,304
Operating Segments | Wealth Management
Segment Reporting Information [Line Items]
Interest income									0	0	0
Interest expense									0	0	0
Net Interest Income									0	0	0
Provision for credit losses									0	0	0
Non-interest income									49	46	44
Non-interest expense									35	34	33
Amortization of intangibles									0	0	1
Income tax expense (benefit)									3	3	2
Net income (loss)									11	9	8
Total assets	38				32				38	32	26
Total intangibles	9				10				9	10	10
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Interest income									0	0	0
Interest expense									0	0	0
Net Interest Income									0	0	0
Provision for credit losses									0	0	0
Non-interest income									22	20	16
Non-interest expense									19	17	17
Amortization of intangibles									1	1	0
Income tax expense (benefit)									0	0	0
Net income (loss)									2	2	(1)
Total assets	35				35				35	35	25
Total intangibles	28				28				28	28	20
Operating Segments | Consumer Finance
Segment Reporting Information [Line Items]
Interest income											25
Interest expense											2
Net Interest Income											23
Provision for credit losses											6
Non-interest income											2
Non-interest expense											15
Amortization of intangibles											0
Income tax expense (benefit)											1
Net income (loss)											3
Total assets											0
Total intangibles											0
Parent and Other
Segment Reporting Information [Line Items]
Interest income									1	2	0
Interest expense									20	20	17
Net Interest Income									(19)	(18)	(17)
Provision for credit losses									0	0	1
Non-interest income									(10)	(9)	1
Non-interest expense									8	10	5
Amortization of intangibles									0	0	0
Income tax expense (benefit)									(7)	(7)	(6)
Net income (loss)									(30)	(30)	(16)
Total assets	36				57				36	57	54
Total intangibles	$ 0				$ 0				$ 0	$ 0	$ 0