Investments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Total Investments [Line Items]
Current investments	$ 2,462	$ 2,522
Long-term investments	16,973	15,732
Total investments	19,435	18,254
Debt securities available for sale
Total Investments [Line Items]
Current investments	2,270	2,359
Long-term investments	14,199	12,896
Total investments	16,469	15,255
Mortgage loans
Total Investments [Line Items]
Current investments	175	145
Long-term investments	1,170	1,216
Total investments	1,345	1,361
Other investments
Total Investments [Line Items]
Current investments	17	18
Long-term investments	1,604	1,620
Total investments	$ 1,621	$ 1,638