Investments, Contract Maturity of Available-for-Sale Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Company	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Contract Maturity of Available-for-Sale Securities [Abstract]
One year or less	55.00%
After one year but within two years	32.00%
After two years but within three and a half years	13.00%
Total	100.00%
Percentage of available-for-sale securities with a maturity of less than two years	87.00%
Ownership Interests in Private and Public Companies [Abstract]
Number of privately-held companies in which there is an equity ownership interest of less than 20% | Company	3
Number of publicly-held companies in which there is an equity ownership interest of less than 20% | Company	2
Gain on investments, net		$ 20,300	$ 21,200
Gain on investment in Regulus Therapeutics Inc.	$ 0	20,211	$ 19,902
Investment in Regulus Therapeutics Inc.	$ 2,414	$ 24,792