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Investments, Contract Maturity of Available-for-Sale Securities (Details)
9 Months Ended
Sep. 30, 2019
Company
Contract Maturity of Available-for-Sale Securities [Abstract]  
One year or less 69.00%
After one year but within two years 22.00%
After two years but within three years 9.00%
Total 100.00%
Percentage of available-for-sale securities with a maturity of less than two years 91.00%
Maximum contract maturity period, range 1 1 year
Maximum contract maturity period, range 2 2 years
Maximum contract maturity period, range 3 3 years
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% 2