Coming back from Key West Fl. We were flying in a commercial A/C , flying at 12,000 ft lookd down at [45 deg] out the right window near me in cloud bank noticed the foot ball shaped object floating among the cloud's. It was just going south with the clouds drifting with them sorta looked like a blimp at first but as we observed it it seemed to have no markings just a football shaped white thing ,could not tell size but may be 25ft to 50 ft? we were over the outer fringes of the southern Florida area and lots of swamps and wet land below us. It was very clear that day. The clouds were big and fluffy and numerous with spaces between them ,That is were we spotted this object. No lights, no markings, no wings, It was about 2:00pm we were on a American Airlines flight to Miami from Key West . The clouds were app. 1000 ft off the ground.