If you leave your present employment for any reason, Federal Law requires your health insurance company to offer you the same or similar policy for 18 months.
Of course your employer does not contribute his share anymore so the premiums will be higher. This allows you to have no break in your health insurance so you cannot be denied coverage for preexisting conditions if or when you resume employment.
These are only the highlights, for more info:
http://www.dol.gov/dol/topic/health-plans/cobra.htm
Regards
Joe