| Document and Entity Information | Jul. 27, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 27, 2020 |
| Entity Registrant Name | COASTAL FINANCIAL CORP |
| Entity Central Index Key | 0001437958 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity File Number | 001-38589 |
| Entity Incorporation State Country Code | WA |
| Entity Tax Identification Number | 56-2392007 |
| Entity Address Address Line1 | 5415 Evergreen Way |
| Entity Address City Or Town | Everett |
| Entity Address State Or Province | WA |
| Entity Address Postal Zip Code | 98203 |
| City Area Code | 425 |
| Local Phone Number | 257-9000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, no par value per share |
| Trading Symbol | CCB |
| Name of each exchange on which registered | NASDAQ |
