. 2022 $ 424 million of the debentures have a maturity of april 1 , 2020 . these debentures are not subject to redemption prior to maturity . interest is payable semiannually in april and october for both tranches and neither tranche is subject to sinking fund requirements . we subsequently entered into interest rate swaps on the 2020 debentures , which effectively converted the fixed interest rates on the debentures to variable libor-based interest rates