| Document and Entity Information - USD ($) $ in Millions | Dec. 31, 2019 | Feb. 24, 2020 | Jun. 28, 2019 |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Amendment Description | TrustCo Bank Corp NY (the “Company”), is filing this Amendment No. 1 (“Amendment No. 1”) to its Annual Report on Form 10-K for the year ended December 31, 2019 (the “Original Filing”) for the sole purpose of filing Exhibit 4.1, which was inadvertently omitted from the Original Filing. | ||
| Document Period End Date | Dec. 31, 2019 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Document Transition Report | false | ||
| Entity Registrant Name | TRUSTCO BANK CORP N Y | ||
| Entity Incorporation, State or Country Code | NY | ||
| Entity File Number | 000-10592 | ||
| Entity Tax Identification Number | 14-1630287 | ||
| Entity Address, Address Line One | 5 SARNOWSKI DRIVE | ||
| Entity Address, City or Town | GLENVILLE | ||
| Entity Address, State or Province | NY | ||
| Entity Address, Postal Zip Code | 12302 | ||
| City Area Code | 518 | ||
| Local Phone Number | 377-3311 | ||
| Entity Well-known Seasoned Issuer | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Filer Category | Large Accelerated Filer | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Emerging Growth Company | false | ||
| Entity Shell Company | false | ||
| Entity Small Business | false | ||
| Entity Public Float | $ 749 | ||
| Entity Common Stock, Shares Outstanding | 96,801,657 | ||
| Document Fiscal Year Focus | 2019 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Central Index Key | 0000357301 | ||
| Document Annual Report | true | ||
| Title of 12(b) Security | Common Stock, $1.00 Par Value | ||
| Trading Symbol | TRST | ||
| Security Exchange Name | NASDAQ |
