If I get health insurance through my employer, what if any information do they have access to?

I'm assuming they don't have access to every doctor's visit, procedure, etc. Do they have access to any information, like what doctors I'm seeing and how much I'm spending through insurance? (I have Blue Cross Blue Shield of Michigan if it makes a difference.)
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Because of the HIPPA privacy laws the employer does not have any access to your private health information.
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