Cash Equivalents and Investments - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,614.2	$ 2,006.5
Restricted cash and cash equivalents			$ 48.7	$ 47.4
Total cash, cash equivalents and restricted cash and cash equivalents	2,645.0	2,059.1	$ 2,290.7	$ 1,880.6
Prepaid expenses and other current assets
Cash and Cash Equivalents [Line Items]
Restricted cash and cash equivalents	27.8	49.6
Other long-term assets
Cash and Cash Equivalents [Line Items]
Restricted cash and cash equivalents	$ 3.0	$ 3.0