It's offered through what is known as the Federal Employee Health Benefit Plan.
They pay for 28% of the insurance out of pocket with the "Agency" paying the remainder. Federal retirees get the same thing.
Postal workers pay 10% ~
You can go to http://www.opm.gov/insure/openseason/index.asp
I was at town hall meeting in Tyler Tx and Rep. Louie Gohmert was asked about Congressman health insurance. His response was everyone thinks Congress gets better health insurance coverage but believe me it lousy insurance.