Coinbase is insured by Aon (who Coinbase states that they are the world's largest insurance broker).   I am not sure if they are the largest or not.

Here is a link to their website:  http://www.aon.com/default.jsp


Again, let me reiterate, that Coinbase has stated that THEY are insured for holding your coins.  YOU are NOT insured against your log-ins being compromised and losing coins as a result...