Release: H.3 Aggregate Reserves of Depository Institutions and the Monetary Base (data not included in press release)
Series is calculated using data from the H.3 release as Total reserve balances maintained (https://fred.stlouisfed.org/series/RESBALNS) less Reserve balance requirements (https://fred.stlouisfed.org/series/RESBALREQ).
Please note though that this historical concept of "excess reserves" no longer has the same meaning following phase two of the simplification of reserves administration. https://federalregister.gov/a/2012-8562
Federal Reserve Bank of St. Louis, Excess Reserves of Depository Institutions [EXCSRESNS], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/EXCSRESNS, March 19, 2019.