John McCain's family has a substantial amount of credit card debt, the Hill reported Friday, after annual Senate financial disclosure reports were made public.

It's not clear exactly how much the McCains owe, as the disclosure forms ask for ranges, not exact numbers. But there are multiple cards involved, according to the Hill -- one held jointly, two in Cindy McCain's name and one for a "dependent child." The joint card has a debt of between $10,000 and $15,000, Cindy McCain's cards both have a debt of between $100,000 and $250,000 and the final card is carrying a debt of between $15,000 and $50,000.

The Huffington Post says that "according to a prior disclosure form filed in May that was provided to The Huffington Post, a significant amount of the McCains' credit card debt is being held by American Express at an interest rate of zero percent."

This is not an unusual practice for American Express, especially when dealing with customers with the kind of high net worth that Cindy McCain has -- instead of collecting interest in these cases, the company charges high annual fees. And, despite appearances, this debt actually seems to make good financial sense for the McCains. In fact, they're probably making money by carrying the debt, because their return from investing the money is almost certainly higher than the interest they're paying.