Federal Employees' Retirement System: Benefits and Financing [Updated November 13, 2019] [open pdf - 1MB]
From the Summary: "Most civilian federal employees who were hired before 1984 are covered by the Civil Service Retirement System (CSRS). Federal employees hired in 1984 or later are covered by the Federal Employees' Retirement System (FERS). Both CSRS and FERS require participants to contribute toward the cost of their pensions through a payroll tax to the Civil Service Retirement and Disability Fund (CSRDF). [...] This report provides an overview of current benefits and financing under CSRS and FERS."
CRS Report for Congress, 98-810
Congressional Research Service: https://crsreports.congress.gov/