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Fair Value Measurements - Fair Value of Convertible Senior Notes (Details) - Senior Notes - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Line Items]
Fair value of outstanding senior convertible notes:	$ 3,075.7	$ 3,155.6
Senior Convertible Notes due 2023
Fair Value Disclosures [Line Items]
Fair value of outstanding senior convertible notes:	1,879.1	1,936.4
Senior Convertible Notes due 2025
Fair Value Disclosures [Line Items]
Fair value of outstanding senior convertible notes:	$ 1,196.6	$ 1,219.2

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[Line Items] for Fair Value Disclosures [Table]

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Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

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