I have mine with Allstate. They required me to have a certain amount of liabilty coverage on my home and auto insurance (I think it was $500k). The umbrella coverage covers claims above what those policies cover. With Allstate the umbrella policies come in $1m increments. Premiums are resonable.... I guess
Not all companies offer the $1M on the base policies, but if yours does it is often less than buying a separate umbrella policy. My insurer, Erie, does offer it.