Regulatory Matters - Schedule of Capital Ratios (Detail) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital, actual amount	$ 3,324	$ 3,174
Tier 1 capital, actual amount	2,759	2,632
Common equity tier 1, actual amount	2,652	2,525
Leverage, actual amount	2,759	2,632
Risk weighted assets, actual amount	$ 26,948	$ 26,866
Total capital, actual ratio	0.1233	0.1181
Total capital, actual ratio tier 1 capital, actual ratio	0.1024	0.0979
Common equity tier 1, actual ratio	9.84%	9.40%
Leverage, actual ratio	0.0783	0.0820
Total capital, well-capitalized requirements amount	$ 2,695	$ 2,687
Tier 1 capital, well-capitalized requirements amount	$ 1,617	$ 1,612
Total capital, well-capitalized requirements ratio	0.1000	0.1000
Tier 1 capital, well-capitalized requirements ratio	0.0600	0.0600
Total capital, minimum capital requirements amount	$ 2,830	$ 2,821
Tier 1 capital, minimum capital requirements amount	2,291	2,284
Common equity tier 1, minimum capital requirements amount	1,886	1,881
Leverage, minimum capital requirements amount	$ 1,410	$ 1,283
Total capital, minimum capital requirements ratio	0.1050	0.1050
Tier 1 capital, minimum capital requirements ratio	0.0850	0.0850
Common equity tier 1, minimum capital requirements ratio	7.00%	7.00%
Leverage, minimum capital requirements ratio	0.0400	0.0400
FNBPA
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital, actual amount	$ 3,400	$ 3,039
Tier 1 capital, actual amount	2,882	2,841
Common equity tier 1, actual amount	2,802	2,761
Leverage, actual amount	2,882	2,841
Risk weighted assets, actual amount	$ 26,902	$ 26,806
Total capital, actual ratio	0.1264	0.1134
Total capital, actual ratio tier 1 capital, actual ratio	0.1071	0.1060
Common equity tier 1, actual ratio	10.42%	10.30%
Leverage, actual ratio	0.0819	0.0887
Total capital, well-capitalized requirements amount	$ 2,690	$ 2,681
Tier 1 capital, well-capitalized requirements amount	2,152	2,144
Common equity tier 1, well-capitalized requirements amount	1,749	1,742
Leverage, well-capitalized requirements amount	$ 1,760	$ 1,601
Total capital, well-capitalized requirements ratio	0.1000	0.1000
Tier 1 capital, well-capitalized requirements ratio	0.0800	0.0800
Common equity tier 1, well-capitalized requirements ratio	6.50%	6.50%
Leverage, well-capitalized requirements ratio	0.0500	0.0500
Total capital, minimum capital requirements amount	$ 2,825	$ 2,815
Tier 1 capital, minimum capital requirements amount	2,287	2,279
Common equity tier 1, minimum capital requirements amount	1,883	1,876
Leverage, minimum capital requirements amount	$ 1,408	$ 1,281
Total capital, minimum capital requirements ratio	0.1050	0.1050
Tier 1 capital, minimum capital requirements ratio	0.0850	0.0850
Common equity tier 1, minimum capital requirements ratio	7.00%	7.00%
Leverage, minimum capital requirements ratio	0.0400	0.0400