Here in Oregon, Ameriprise auto & home insurance is open to non-Costco members, but Costco members get a discount. However, Ameriprise seems to market only through Costco. They used to have very good rates in Oregon, and they are still pretty competitive, the last time I checked.
In CA they seem to have mixed results on the auto product. A coworker lost an auto policy to them so several of us ran our own quotes on their website. Ameriprise was higher for most. This was in comparison to Mercury and MetLife mostly. About 60% higher than my personal MetLife.
Never asked about Costco membership during the quoting process but none of us went any further. Definitely see the advertising while I'm at Costco though.