| Cover Page | Sep. 30, 2021 |
| Cover [Abstract] | |
| Entity Central Index Key | 0000798941 |
| Amendment Flag | false |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | First Citizens BancShares Inc /DE/ |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-16715 |
| Entity Tax Identification Number | 56-1528994 |
| Entity Address, Address Line One | 4300 Six Forks Road |
| Entity Address, City or Town | Raleigh |
| Entity Address, State or Province | NC |
| Entity Address, Postal Zip Code | 27609 |
| City Area Code | 919 |
| Local Phone Number | 716-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Class A [Member] | |
| Cover [Abstract] | |
| Title of 12(b) Security | Class A Common Stock, Par Value $1 |
| Trading Symbol | FCNCA |
| Security Exchange Name | NASDAQ |
| Title of 12(b) Security | Class A Common Stock, Par Value $1 |
| Trading Symbol | FCNCA |
| Security Exchange Name | NASDAQ |
| Series A Preferred Stock [Member] | |
| Cover [Abstract] | |
| Title of 12(b) Security | Depositary Shares, Each Representing a 1/40th Interest in a Share of 5.375% Non-Cumulative Perpetual Preferred Stock, Series A |
| Trading Symbol | FCNCP |
| Security Exchange Name | NASDAQ |
| Title of 12(b) Security | Depositary Shares, Each Representing a 1/40th Interest in a Share of 5.375% Non-Cumulative Perpetual Preferred Stock, Series A |
| Trading Symbol | FCNCP |
| Security Exchange Name | NASDAQ |
