Investments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Total Investments [Line Items]
Current investments	$ 2,426	$ 2,522
Long-term investments	16,410	15,732
Total investments	18,836	18,254
Debt And Equity Securities Available For Sale [Member]
Total Investments [Line Items]
Current investments	2,286	2,359
Long-term investments	13,611	12,896
Total investments	15,897	15,255
Mortgage Loans [Member]
Total Investments [Line Items]
Current investments	123	145
Long-term investments	1,215	1,216
Total investments	1,338	1,361
Other Investments [Member]
Total Investments [Line Items]
Current investments	17	18
Long-term investments	1,584	1,620
Total investments	$ 1,601	$ 1,638