. financing activities cash flows debt . debt increased by $ 1109 million in 2018 , to $ 9347 million at year-end 2018 from $ 8238 million at year-end 2017 , primarily due to the issuance of our series x , y , z , and aa notes , partially offset by the maturity of our series s notes ( $ 330 million ) and lower outstanding commercial paper ( $ 126 million ) . see footnote 10 . long-term debt for additional information on the debt issuances