Walking the dog this morning in the cold, I came by a house with the garage door partially open.
With a car running happily along in the garage filing it with its exhaust, with only some of the exhuast cloud making its way outside and plenty filling in the garage.
The car was running happily for the entire time I walked the dog, over 15 minutes walking past that house and back.
This is really a very bad idea.
Not only bad for your engine, but potentially bad for your health as well. Yes it is cold out there, but a car will warm up soon enough on its own, there's no reason to let it sit running idle in the garage.