Where is The Authority?

Members of Congress were recently asked where congress gets the authority to force Americans to buy insurance. And as yet none really seems to have an answer, Rep. Steny Hoyer claims congress has broad authority under the general welfare clause, “Well, in promoting the general welfare the Constitution obviously gives broad authority to Congress to effect that end,” But this is wrong congress' authority is clearly defined in the enumerated powers, The general welfare is also defined by the constitution, if congress had such broad authority why would the framers have specifically defined the powers of congress, the enumerated powers would be meaningless because congress could do whatever they wanted as long as they thought it was the right thing to do.