Business Segments - Financial Information for Segments of FNB (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 1,005	$ 1,130	$ 1,247
Interest expense	98	208	330
Net Interest Income	907	922	917
Provision for credit losses	1	123	44
Non-interest income	330	294	294
Non-interest expense	721	737	682
Amortization of intangibles	12	13	14
Income tax expense (benefit)	98	57	84
Net income (loss)	405	286	387
Total assets	39,513	37,354	34,615
Total intangibles	2,304	2,316	2,329
Operating Segments | Community Banking
Segment Reporting Information [Line Items]
Interest income	1,003	1,129	1,245
Interest expense	86	188	310
Net Interest Income	917	941	935
Provision for credit losses	(1)	123	44
Non-interest income	251	233	237
Non-interest expense	653	675	621
Amortization of intangibles	11	12	13
Income tax expense (benefit)	100	61	88
Net income (loss)	405	303	406
Total assets	39,396	37,245	34,491
Total intangibles	2,268	2,279	2,291
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	61	49	46
Non-interest expense	39	35	34
Amortization of intangibles	0	0	0
Income tax expense (benefit)	5	3	3
Net income (loss)	17	11	9
Total assets	37	38	32
Total intangibles	9	9	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	24	22	20
Non-interest expense	21	19	17
Amortization of intangibles	1	1	1
Income tax expense (benefit)	0	0	0
Net income (loss)	2	2	2
Total assets	34	35	35
Total intangibles	27	28	28
Parent and Other
Segment Reporting Information [Line Items]
Interest income	2	1	2
Interest expense	12	20	20
Net Interest Income	(10)	(19)	(18)
Provision for credit losses	2	0	0
Non-interest income	(6)	(10)	(9)
Non-interest expense	8	8	10
Amortization of intangibles	0	0	0
Income tax expense (benefit)	(7)	(7)	(7)
Net income (loss)	(19)	(30)	(30)
Total assets	46	36	57
Total intangibles	$ 0	$ 0	$ 0