Investments, Contract Maturity of Available-for-Sale Securities (Details) shares in Millions, $ in Millions	1 Months Ended
	Jul. 31, 2015 USD ($) shares	Sep. 30, 2015 Company
Contract Maturity of Available-for-Sale Securities [Abstract]
One year or less		52.00%
After one year but within two years		28.00%
After two years but within three and a half years		20.00%
Total		100.00%
Percentage of available-for-sale securities with a maturity of less than two years		80.00%
Ownership Interests in Private and Public Companies [Abstract]
Number of privately-held companies in which the entity has an equity ownership interest of less than 20% | Company		1
Number of publicly-held companies in which the entity has an equity ownership interest of less than 20% | Company		2
Regulus [Member]
Ownership Interests in Private and Public Companies [Abstract]
Number of shares of common stock sold (in shares) | shares	2.7
Total proceeds from sale of common stock	$ 25.5
Gain from sale of common stock	$ 20.2