Cash Equivalents and Investments - Textuals (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Cash Equivalents and Investments [Abstract]
Total investments In unrealized loss position | Investment	494	682
Other than temporarily impaired	$ 0	$ 0	$ 1,100,000
Available-for-sale securities, gross realized gains			166,800,000
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments, current	119,200,000
Privately held investments at carrying value	62,700,000	102,400,000
Privately held investments at fair value	43,700,000	60,200,000
Unrealized gain on privately held investments		11,400,000	15,000,000
Other than temporary impairment of privately held equity investments	11,200,000	$ 0	$ 1,100,000
Prepaid Expenses and Other Current Assets [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments, current	19,300,000
Restricted Cash and Investments, Noncurrent [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments, current	$ 99,900,000