Cash Equivalents and Investments - Restricted Cash and Investments (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 986.7	$ 1,361.9
Total cash, cash equivalents, and restricted cash	1,009.4	1,383.0	$ 1,521.2	$ 1,276.5
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in Prepaid expenses and other current assets	19.0	19.2
Other long-term assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in Prepaid expenses and other current assets	$ 3.7	$ 1.9