Principalization of ideals in abelian extensions of number fields

We give the complete proof of a conjecture of Georges Gras which claims that, for any extension $K\mathrm{/}k$ of number fields in which at least one infinite place is totally split, every ideal $I$ of $K$ principalizes in the compositum $K{k}^{ab}$ of $K$ with the maximal abelian extension $k^{ab}$ of $k$