Item 3. Quantitative and Qualitative Disclosures About Market Risk.

We use derivative instruments to manage foreign exchange risk on the Sterling Notes, and do not hold or issue derivative instruments for speculative trading purposes.

Cross-currency derivative instruments are used to effectively convert £1.275 billion aggregate principal amount of fixed-rate British pound sterling denominated debt, including annual interest payments and the payment of principal at maturity, to fixed-rate U.S. dollar denominated debt. The cross-currency derivative instruments have maturities of June 2031 and July 2042. We are required to post collateral on the cross-currency derivative instruments when such instruments are in a liability position. In April 2019, we entered into a collateral holiday agreement for 60% of both the 2031 and 2042 cross-currency swaps, which eliminates the requirement to post collateral for three years, as well as a ten year collateral cap on the remaining 40% of the cross-currency swaps which limits the required collateral posting on that 40% of the cross-currency swaps to $150 million. In March 2021, the collateral holiday for 20% of the swaps was extended to November 2022 in consideration for our agreement to post collateral over a threshold amount on that 20% portion of the swaps from March 2021 through October 2021. The fair value of our cross-currency derivatives included in other long-term liabilities on our consolidated balance sheets was $317 million and $184 million as of September 30, 2021 and December 31, 2020, respectively. For more information, see Note 9 to the accompanying consolidated financial statements contained in “Item 1. Financial Statements.”

As of September 30, 2021 and December 31, 2020, the weighted average interest rate on credit facility debt was approximately 1.6% and 1.7%, respectively, and the weighted average interest rate on the senior notes was approximately 5.0% and 5.1%, respectively, resulting in a blended weighted average interest rate of 4.5% and 4.7%, respectively. The interest rate on approximately 86% and 87% of the total principal amount of our debt was effectively fixed as of September 30, 2021 and December 31, 2020, respectively.

The table set forth below summarizes the fair values and contract terms of financial instruments subject to interest rate risk maintained by us as of September 30, 2021 (dollars in millions).

Interest rates on variable-rate debt are estimated using the average implied forward LIBOR for the year of maturity based on the yield curve in effect at September 30, 2021 including applicable bank spread.