I don’t believe this happened 15 years ago, I think it’s a lot more recent.
In his senate years, Ted "lives" in Washington DC and Heidi in Texas. At times before that, it was reverse---Ted lived in Texas as Solicitor General and Heidi in DC working for Bush. So when the cat's away...He would have had plenty of opportunity.
Google around or look through my previous posts---I have posted a whole lot of info and links on Heidi---there's an interview with Heidi where she says the daughters refer to Ted, when he comes home from Washington for the weekend or whatever, "the guest."
Heidi has said she has a nanny, maybe more than one, so probably during this campaign she's maybe able to keep an eye on Ted's roving eye more.