Table:
|  |  | At December 31, 2019 |  |
| --- | --- | --- | --- |
|  | Gross Carrying Amount | Unamortized Discount and Deferred Financing Costs | Net Carrying Amount |
| 2021 Notes | $650 | $(2) | $648 |
| 2022 Notes | 400 | (2) | 398 |
| 2026 Notes | 850 | (7) | 843 |
| 2027 Notes | 400 | (5) | 395 |
| 2047 Notes | 400 | (9) | 391 |
| Total long-term debt | $2,700 | $(25) | $2,675 |
|  |  | At December 31, 2018 |  |
|  | Gross Carrying Amount | Unamortized Discount and Deferred Financing Costs | Net Carrying Amount |
| 2021 Notes | 650 | (3) | 647 |
| 2022 Notes | 400 | (3) | 397 |
| 2026 Notes | 850 | (8) | 842 |
| 2027 Notes | 400 | (5) | 395 |
| 2047 Notes | 400 | (10) | 390 |
| Total long-term debt | $2,700 | $(29) | $2,671 |
Paragraphs:
Interest expense and financing costs
Fees and discounts associated with the issuance of our debt instruments are recorded as debt discount, which reduces their respective carrying values, and are amortized over their respective terms. Amortization expense is recorded within “Interest and other expense (income), net” in our consolidated statement of operations.
For the years ended December 31, 2019, 2018, and 2017: interest expense was $86 million, $134 million, and $150 million, respectively; amortization of the debt discount and deferred financing costs was $4 million, $6 million, and $12 million, respectively.
A summary of our outstanding debt is as follows (amounts in millions):
With the exception of the 2026 and the 2047 Notes, using Level 2 inputs (i.e., observable market prices in less-than-active markets) at December 31, 2019, the carrying values of the Notes approximated their fair values, as the interest rates were similar to the current rates at which we could borrow funds over the selected interest periods. At December 31, 2019, based on Level 2 inputs, the fair value of the 2026 and the 2047 Notes were $893 million and $456 million, respectively.
Using Level 2 inputs at December 31, 2018, the carrying values of the 2021 Notes and the 2022 Notes approximated their fair values, as the interest rates were similar to the current rates at which we could borrow funds over the selected interest periods. At December 31, 2019, based on Level 2 inputs, the fair values of the 2026 Notes, the 2027 Notes, and the 2047 Notes were $800 million, $376 million, and $360 million, respectively.
Question:
What was the percentage change in the net carrying amount of total long-term debt between 2018 and 2019?
Answer: ($2675-$2671)/$2671 = 0.15