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Debt (Tables) | 12 Months Ended
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Dec. 31, 2021
Debt Disclosure [Abstract] |
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments | The carrying amounts of our senior convertible notes were as follows as of the dates indicated:
| December 31,
(Dollars in millions) | 2021 |  | 2020
Principal amount: |  |  |
Senior Convertible Notes due 2023 | $ | 792.3 |  |  | $ | 850.0 |
Senior Convertible Notes due 2025 | 1,207.5 |  |  | 1,207.5 |
Total principal amount | 1,999.8 |  |  | 2,057.5 |
Unamortized debt discount | (282.7) |  |  | (371.1) |
Unamortized debt issuance costs | (14.4) |  |  | (19.2) |
Carrying amount of liability component | $ | 1,702.7 |  |  | $ | 1,667.2 |
Carrying value of equity component | $ | 449.4 |  |  | $ | 461.0 |
Remaining amortization period of debt discount on the liability component: |  |  |
Senior Convertible Notes due 2023 | 1.9 years |  | 2.9 years
Senior Convertible Notes due 2025 | 3.9 years |  | 4.9 years
Schedule of Converted Value of Notes | For our senior convertible notes for which the if-converted value exceeded the principal amount, the amount in excess of principal is as follows as of the dates indicated:
| December 31,
(In millions) | 2021 |  | 2020
Senior Convertible Notes due 2023 | $ | 1,797.3 |  |  | $ | 1,077.5 |
Senior Convertible Notes due 2025 | 141.8 |  |  | — |
Total by which the notes’ if-converted value exceeds their principal amount | $ | 1,939.1 |  |  | $ | 1,077.5 |
Schedule of Components of Interest Expense and Effective Interest Rates of Senior Convertible Notes | The following table summarizes the components of interest expense and the effective interest rates for each of our senior convertible notes for the periods shown.
| Twelve Months Ended December 31,
(Dollars in millions) | 2021 |  | 2020 |  | 2019
Cash interest expense: |  |  |  |  |
Contractual coupon interest (1)(2) | $ | 9.3 |  |  | $ | 9.3 |  |  | $ | 9.3 |
Non-cash interest expense: |  |  |  |  |
Accretion of debt discount | 83.0 |  |  | 68.6 |  |  | 45.8 |
Amortization of debt issuance costs | 4.4 |  |  | 4.0 |  |  | 3.7 |
Total interest expense recognized on senior notes | $ | 96.7 |  |  | $ | 81.9 |  |  | $ | 58.8 |
Effective interest rates: |  |  |  |  |
Senior Convertible Notes due 2022 (2) | * |  | 5.1 | % |  | 5.1 | %
Senior Convertible Notes due 2023 | 5.6 | % |  | 5.6 | % |  | 5.6 | %
Senior Convertible Notes due 2025 | 5.5 | % |  | 5.5 | % |  | *
(1) Interest on the 2022 Notes began accruing upon issuance and was payable semi-annually on May 15 and November 15 of each year. Interest on the 2023 Notes began accruing upon issuance and is payable semi-annually on June 1 and December 1 of each year. Interest on the 2025 Notes began accruing upon issuance and is payable semi-annually on May 15 and November 15 of each year. (2) The effective interest rate presented represents the rate applicable for the period outstanding. Our $400.0 million aggregate principal amount of unsecured senior convertible notes issued in June 2017 with a stated interest rate of 0.75% and maturity date of May 15, 2022 (the 2022 Notes) were repurchased and converted by August 2020. * Not applicable as no notes were outstanding at this date.
Schedule of Availability and Outstanding Borrowings on Credit Agreement | Information related to availability and outstanding borrowings on our Amended Credit Agreement is as follows as of the date indicated:
| December 31,
(In millions) | 2021
Available principal amount | $ | 200.0 |
Letters of credit sub-facility | 25.0 |
Outstanding borrowings | — |
Outstanding letters of credit | 7.3 |
Total available balance | $ | 192.7 |

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- Definition Tabular disclosure of short-term or long-term contractual arrangements with lenders, including letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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