The IRS says
"The CSRS (Civil Service Retirement System), FERS (Federal Employees Retirement System), and TSP (Thrift Savings Plans) annuities are considered qualified retirement plans. You can find information about computing the taxable portion of your annuity by going to IRS Publication 721 (Tax Guide to U.S. Civil Service Retirement Benefits) on the Internal Revenue Service website."
From (https://www.opm.gov/faqs/QA.aspx?fid=735eda40-61a8-45df-b6ad-47185f4c91a5&pid=dcdd4c76-cea6-4e81-89e...)
