pre_text: The below tables represents the key components of Teradyne’s convertible senior notes: As of December 31, 2019, the unamortized discount was $65.3 million, which will be amortized over four years using the effective interest rate method. The carrying amount of the equity component was $100.8 million. As of December 31, 2019, the conversion price was approximately $31.62 per share and if converted the value of the notes was $992.0 million.
table: December 31, 2019 December 31, 2018
table: (in thousands)
table: Debt principal $460,000 $460,000
table: Unamortized discount 65,313 80,019
table: Net carrying amount of convertible debt $394,687 $379,981
question: Analyse this data from a financial earnings document. What was the change in Debt principal from 2018 to 2019?