I'm positive it's Agaricus campestris.
There's a couple of mushrooms it can be mistaken:
1. Amanita phalloides, but the gills are white and it has a volva at the base;
2. Agaricus xanthoderma, but that one should have funny smell, and the stipe/stalk turns to yellow quick when wounded.

Heh, I found an Agaricus bernardii.