Medicare coverage with no spouse and two other private insurance policies who pays first

[FlyguyAtl]

Greetings, I have Medicare coverage as of this past Summer. I also have coverage via a domestic partner relationship, but we are not married and this is with Cigna. I also have the AARP United healthcare supplement Insurance. I am not married and only work part time. I have no insurance from a previous employer and again there is no marriage and no coverage from "spouse".

How does medicare determine who is first, second, and third payer. It seems it causes a great deal of confusion.  Thank you for any advice.  

[GailL1]

In don’t know anything specific or special about Cigna - 

Who pays 1st is based on the number of employees that are insured under your partner’s EHB 

Medicare.gov - Working past 65 

Most people, if they have a large employer (over 20) and the EHB is a good plan,  don’t opt for a Medigap plan - IOW you wouldn’t have to sign up for Part B coverage until the EHB is ending - that saves your Medicare Part B premiums and the cost of the Medigap plan.  Remember that is for large employers.

So what size is the employer?  If you don’t know this or the Medicare credible nature of the policy, check with HR.  This is especially important in the prescription drug benefit (Medicare Part D).

Before I can try to answer your question - what size is the employer.  (See the link) cause this makes all the difference in who pays what and whem.

[FlyguyAtl]

Thank you for your information. The company is a large one with thousands of employees. The issue is that we are not married. Medicare rules concerning insurance are specific to "spouse" and they do not appear to recognize "Domestic Partner". This is the reason that I could not delay signing up for Medicare coverage at 65 even with existing coverage from my partner's employer. I am sure it will work out, but Medicare switches from being primary to secondary and back to primary depending on who I speak with.