Fair Value Measurements - Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value
Cash equivalents	$ 6,004	$ 5,755
Restricted cash equivalents	1,436	1,747
Short-term investments	5,575	5,789
Long-term investments	1,673	1,417
Level 1
Fair Value
Cash equivalents	6,004	5,755
Restricted cash equivalents	1,436	1,747
Short-term investments	3,564	3,919
Long-term investments	895	948
Level 2
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Short-term investments	2,011	1,870
Long-term investments	557	38
Level 3
Fair Value
Cash equivalents	0	0
Restricted cash equivalents	0	0
Short-term investments	0	0
Long-term investments	$ 221	$ 431