My friend is apparently really sensitive...?

I'm a pretty witty and sarcastic person. I guess I ume that people know not to take me seriously when I sarcastically "insult" them. So, I was chatting with my friend online and we were having a convo about what we were doing that day.. just a regular sarcastic convo.. He invited me to a movie with friends, and I said maybe.. then he said "fine" and I said Fine too and then asked him a question of some kind... Then I called him a "P90x idiot" (he is starting that P90x workout video soon). I was just being retarded and goofy, like usual. He said "idiot? Umm goodbye.. have a good day" Then he signed off, and I didn't talk to him for most of the day.. I said that I was just joking, and didn't mean that he was seriously an idiot... I laughed about it because I couldn't believe that he was being so serious about something so minor and ridiculous.. He was pretty much ignoring me even after I apologized.. He said that it was rude, and calling him an idiot offended him.. I told him that he insults me alll the time, and I get offended then I get over it, because I know it's all in fun and he isn't truly trying to make me feel bad.. That being said, I just said that I was sorry, and I say that to everybody.. I didn't mean for you to take it the wrong way, and I won't use that word again.." Now, I just feel weird.. I can't believe that he took it so seriously.. I don't feel like I can just joke around with him like normal now.. it's annoying. Like, what the hell? Now do I have to think twice before being myself around him?? He's usually great at not taking things personally.. I mean he does the same thing to me ,but I guess I crossed a "line".. ugh whatever.. I feel awkward now...hmmmm...