Table:
| ($ in millions) |  |  |
| --- | --- | --- |
| At December 31: | 2019 | 2018 |
| Noncurrent assets | $113,767 | $74,236 |
| Long-term debt | $ 54,102 | $35,605 |
| Noncurrent liabilities (excluding debt) | $ 39,398 | $32,621 |
Paragraphs:
Noncurrent Assets and Liabilities
The increase in noncurrent assets of $39,531 million ($39,470 million adjusted for currency) was driven by: • A net increase in goodwill and net intangible assets of $34,104 million ($34,058 million adjusted for currency) due to the acquisition of Red Hat; and • An increase in operating right-of-use assets of $4,996 million ($5,010 million adjusted for currency) as a result of the adoption of the new leasing standard on January 1, 2019; and• An increase in prepaid pension assets of $2,199 million ($2,152 million adjusted for currency) driven by higher returns on plan assets and plan remeasurements; partially offset by • A decrease in net property, plant and equipment of $782 million ($785 million adjusted for currency).
Long-term debt increased $18,497 million ($18,550 million adjusted for currency) primarily driven by: • Issuances of $26,081 million; partially offset by • Reclassifications to short-term debt of $7,592 million to reflect upcoming maturities.
Noncurrent liabilities (excluding debt) increased $6,778 million ($6,911 million adjusted for currency) primarily driven by: • An increase in long-term operating lease liabilities of $3,879 million ($3,893 million adjusted for currency) as a result of the adoption of the new leasing standard on January 1, 2019; and • An increase in other liabilities of $2,352 million ($2,320 million adjusted for currency), primarily driven by increases in deferred tax liabilities of $1,534 million and income tax reserves of $923 million.
Question:
What caused the increase in the prepaid expenses?