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Debt (Tables)
 9 Months Ended
Sep. 30, 2021
Debt Disclosure [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments
The fair value, based on trading prices (Level 1), of our senior convertible notes were as follows as of the dates indicated:
Fair Value Measurements Using Level 1
(In millions)September 30, 2021December 31, 2020
Senior Convertible Notes due 2023$2,719.0 $1,936.4 
Senior Convertible Notes due 20251,432.2 1,219.2 
Total fair value of outstanding senior convertible notes$4,151.2 $3,155.6 
The carrying amounts of our senior convertible notes were as follows as of the dates indicated:
(Dollars in millions)September 30, 2021December 31, 2020
Principal amount:
Senior Convertible Notes due 2023$817.4 $850.0 
Senior Convertible Notes due 20251,207.5 1,207.5 
Total principal amount2,024.9 2,057.5 
Unamortized debt discount(305.7)(371.1)
Unamortized debt issuance costs(15.7)(19.2)
Carrying amount of liability component$1,703.5 $1,667.2 
Carrying amount of equity component$454.5 $461.0 
Remaining amortization period of debt discount on the liability component:
Senior Convertible Notes due 20232.2 years2.9 years
Senior Convertible Notes due 20254.1 years4.9 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 was as follows as of the dates indicated:
(In millions)September 30, 2021December 31, 2020
Senior Convertible Notes due 2023$1,903.6 $1,077.5 
Senior Convertible Notes due 2025163.9 —
Total by which the notes’ if-converted value exceeds their principal amount
$2,067.5 $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.

Three Months Ended
September 30,		Nine Months Ended
September 30,
(Dollars in millions)2021202020212020
Cash interest expense:
Contractual coupon interest (1),(2)
$2.4 $2.4 $7.0 $7.1 
Non-cash interest expense:
Accretion of debt discount 20.8 20.0 62.1 48.3 
Amortization of debt issuance costs1.1 1.1 3.3 3.1 
Total interest expense recognized on senior notes$24.3 $23.5 $72.4 $58.5 
Effective interest rates:
Senior Convertible Notes due 2022 (2)
*5.1 %*5.1 %
Senior Convertible Notes due 20235.6 %5.6 %5.6 %5.6 %
Senior Convertible Notes due 20255.5 %5.5 %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) 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 Credit Agreement is as follows as of the date indicated:
(In millions)September 30, 2021
Available principal amount $200.0 
Letters of credit sub-facility10.0 
Outstanding borrowings — 
Outstanding letters of credit6.4 
Total available balance$193.6