Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Total Investments [Line Items]
Current investments	$ 2,334	$ 2,522
Long-term investments	17,342	15,732
Total investments	19,676	18,254
Debt securities available for sale
Total Investments [Line Items]
Current investments	2,175	2,359
Long-term investments	14,583	12,896
Total investments	16,758	15,255
Mortgage loans
Total Investments [Line Items]
Current investments	159	145
Long-term investments	1,131	1,216
Total investments	1,290	1,361
Other investments
Total Investments [Line Items]
Current investments	0	18
Long-term investments	1,628	1,620
Total investments	$ 1,628	$ 1,638