https://www.ssa.gov/policy/docs/chartbooks/fast_facts/2019/f...
https://www.ssa.gov/agency/performance/
And I don't know of a debt instrument that can be exited more easily than US Treasuries:
https://www.ssa.gov/news/press/factsheets/WhatAreTheTrust.ht...
This interview with a prominent actuary who worked in the Social Security admin is also very helpful to clarify the workings of SS:
https://us14.campaign-archive.com/?u=bd0d1b66f832083794c33c9...