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Stockholders' Equity - Stock Options - Grant Date Fair Value of Options Vested (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]      
Fair value of options to purchase common stock vested $ 9.3 $ 2.8 $ 3.7