Investments, Contract Maturity of Available-for-Sale Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Company	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Contract Maturity of Available-for-Sale Securities [Abstract]
One year or less	71.00%
After one year but within two years	23.00%
After two years but within three and a half years	6.00%
Total	100.00%
Percentage of available-for-sale securities with a maturity of less than two years	94.00%
Maximum contract maturity period, range 1	1 year
Maximum contract maturity period, range 2	2 years
Maximum contract maturity period, range 3	3 years 6 months
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%	5
Number of publicly-held companies in which there is an equity ownership interest of less than 20%	1
Gain on investment in Regulus Therapeutics Inc. | $	$ 374	$ 0	$ 20,211