If my employer offers health insurance to its employee, is it required to offer this benefit to all employees?

Health insurance for full time employees, is the employer required to offer insurance to every full time employee?
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All of the same class of employees. We have union and non union and only give insurance to the non union. We pay the union employees a high wage and they divert several dollars an hour to the unions to pay for medical, apprenticeships, short term disability and retirement. Each month I write a large check to each union for the diversions they run around $8 an hour some less depending what that union voted on.
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