Investments, Contract Maturity of Available-for-Sale Securities (Details)	6 Months Ended
	Jun. 30, 2018 Company
Contract Maturity of Available-for-Sale Securities [Abstract]
One year or less	81.00%
After one year but within two years	16.00%
After two years but within three and a half years	3.00%
Total	100.00%
Percentage of available-for-sale securities with a maturity of less than two years	97.00%
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%	4
Number of publicly-held companies in which there is an equity ownership interest of less than 20%	1