Not sure if it would include military - I thought it was just civilian. From OPM “every federal agency”. I know they do DOD civilians. I’m also not sure about the Post Office - technically not an agency, but I know they are considered government employees.
“As the central human resources planners for the Federal Government, OPM is responsible for the successful management of human capital, not only within our own organization, but also across every Federal agency. We assist Federal agencies in hiring new employees, provide Federal investigative services for background checks, create training programs to develop tomorrow’s leaders — and much more.”
OPM's website mentions 4 million hacked. Federal civilian employee numbers are listed at 2,663,000 and military at 1,459,000 for 2014. Legislative and judicial branch at 63,000. That's a current sum of 4,185,000. Sources: http://www.opm.gov/policy-data-oversight/data-analysis-documentation/federal-employment-reports/historical-tables/total-government-employment-since-1962/
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