Source please?
Here’s a list of where the members get their insurance.
It’s a little out of date. For example, Ted Cruz had to sign up for Obamacare when he lost his wife’s coverage.
http://www.cnn.com/2015/03/24/politics/ted-cruz-obamacare/index.html
The only “exemption” they get is that they get a stipend to pay for it. Employers are supposed to offer insurance under the ACA so they can’t pay for exchange plans, but since congress and their staff are required to buy their insurance on the exchanges, they were given a special rule to allow them to treat it like an employer sponsored plan.
But the plan they get is bought on the exchange.