Standard pronunciation is 'thee' idea, not 'thuh' idea.
Before vowels, say 'thee'.
What is really to the point here is that there must at least be an intrusion of a /j/ sound (like the first sound in yes) between the two words, in order to allow a flowing connection. In actual fact, in normally paced speech, it can barely be discriminated between thuyidea and theeyidea.
But having said that, the normal 'rule' to teach learners is to use thee before vowels.