Cash Equivalents and Investments - Equity securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity investments without readily determinable fair value	$ 30.4
Total equity securities	1,640.4
Equity investments without readily determinable fair value		$ 29.7
Total equity securities		1,027.1
Mutual funds
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity investments with readily determinable fair value	25.9
Equity investments with readily determinable fair value		27.6
Money market funds
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity investments with readily determinable fair value	1,584.1
Equity investments with readily determinable fair value		969.8
Cash equivalents
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Total equity securities	1,542.3
Total equity securities		928.0
Prepaid expenses and other current assets
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Total equity securities	34.6
Total equity securities		36.3
Other long-term assets
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Total equity securities	$ 63.5
Total equity securities		$ 62.8