Fair Value Measurements - Fair Value of Convertible Senior Notes (Details) - Senior Notes - USD ($) $ in Millions |
Mar. 31, 2020 |
Dec. 31, 2019 |
---|---|---|
Fair Value Disclosures [Line Items] | ||
Fair value of outstanding senior convertible notes: | $ 2,562.9 | $ 2,150.8 |
0.75% Senior Convertible Notes due 2022 | ||
Fair Value Disclosures [Line Items] | ||
Fair value of outstanding senior convertible notes: | 1,088.9 | 890.8 |
0.75% Senior Convertible Notes due 2023 | ||
Fair Value Disclosures [Line Items] | ||
Fair value of outstanding senior convertible notes: | $ 1,474.0 | $ 1,260.0 |