How about "insouciant"insouciant? The simple definition on Merriam-Webster [http://www.merriam-webster.com/dictionary/insouciant] fits pretty well:
"a relaxed and calm state : a feeling of not worrying about anything"
"a relaxed and calm state : a feeling of not worrying about anything"
So does the full definition:
"lighthearted unconcern : nonchalance"
"lighthearted unconcern : nonchalance"
M-W's example is: "wandered into the meeting with complete insouciance to the fact that she was late."
"wandered into the meeting with complete insouciance to the fact that she was late."