It's a $3 trillion tax cut over 10 years. They get to $1B by finding an "offset" of $2 billion, which assumes a very optimistic growth estimate, which there was political push to develop. In particular, that offset assumes there is no bubble to pop.

Good luck to the US. The yearly budget is $3.8T. Of that, only $1B is discretionary. $3T of tax cuts is three years of the entire discretionary budget. The cut is huge. And that cut wasn't Keynesian stimulus - it came during a good economy. Good luck to the US.