Is Buddhism a kind of philosophy? No. There are Eastern, Western and other types of philosophy. Some of them may enunciate certain thoughts that are similar to that of Buddhism, but their analyses never go as deep. Hence, Buddhism is not a branch of philosophy.

Is it science? Certain views of Buddhism and some findings of science may be the same, but Buddhism as a whole is not science.

Could it be idealism? Many people consider religion idealism. It may be the case in terms of Western religions. As most philosophers in the West are idealists, albeit holding different philosophical positions, they simply identify religion as a category of idealism as well. However, Buddhist thought and idealism are completely different.

~Depicted from THE RIGHT VIEW - Buddhism—the Definition