I thought that $18 Trillion was the market cap for the NYSE. Are those two numbers coincidentally the same number?
Wait, I decided not to be lazy and to look it up. US National debt is $20.5 trillion. US GDP is $18.5 trillion. NYSE market cap is $21.0 trillion.
All numbers are fairly similar. Also, all numbers are approximate. But who's counting, when you're throwing around $500 billion here and there?