Cash Equivalents and Investments - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) Investment	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Investment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Investment
Cash Equivalents and Investments [Abstract]
Total investments in unrealized loss position | Investment	517		517		494
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	$ 115.5		$ 115.5
Investments in privately-held companies	72.5		72.5		$ 62.7
Cost method investment	30.2		30.2		$ 19.0
Prepaid expenses and other current assets [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	52.1		52.1
Restricted Cash and Investments, Noncurrent [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	63.4		63.4
Debt Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI	0.0	$ 0.0	0.0	$ 0.0
Equity Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI	0.0	0.0	0.0	0.0
Certain Investments In Privately-Held Companies [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Impairment of privately-held investment	$ 0.0	$ 0.0	$ 0.0	$ 5.1