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Stockholders' Equity - Stock Options - Intrinsic Value of Options Exercised (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]      
Total intrinsic value of options exercised $ 79.6 $ 50.8 $ 59.0