Cash Equivalents, and Investments - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Investment	Sep. 30, 2012	Sep. 30, 2013 Investment	Sep. 30, 2012	Dec. 31, 2012 Investment
Cash Equivalents and Investments [Abstract]
Total investments in unrealized loss position	230		230		98
Privately-held investments	$ 36.5		$ 36.5		$ 32.0
Cost method investment, amount reclassified to short-term investments subsequent to investee initial public offering	7.6
Unrealized gain on cost method investments recorded upon investee initial public offering	100.0		100.0
Privately-held investments, other than temporary impairment	$ 2.1	$ 6.0	$ 2.5	$ 20.0