Source: Board of Governors of the Federal Reserve System (US)  

Release: H.4.1 Factors Affecting Reserve Balances (data not included in press release)

Units:  Billions of Dollars, Not Seasonally Adjusted

Frequency:  Monthly

Data prior to 1991 are from various issues of Banking and Monetary Statistics and the Annual Statistical Digest. Data from 1991 forward are calculated using data from the H.4.1 Release - Factors Affecting Reserve Balances.
As of March 1, 2019, this series is calculated as a monthly average of fred.stlouisfed.org/series/WCURCIR.