Investments, Contract Maturity of Available-for-Sale Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Company	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Contract Maturity of Available-for-Sale Securities [Abstract]
One year or less	49.00%
After one year but within two years	27.00%
After two years but within three and a half years	24.00%
Total	100.00%
Percentage of available-for-sale securities with a maturity of less than two years	76.00%
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
Gain on investments, net	$ 20,300	$ 21,200	$ 2,400
Gain on investment in Regulus Therapeutics Inc.	20,211	19,902	$ 0
Investment in Regulus Therapeutics Inc.	$ 24,792	$ 81,881
Regulus Therapeutics Inc. [Member]
Ownership Interests in Private and Public Companies [Abstract]
Ownership interest percentage	5.00%