Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Within one year	$ 11,292
After one year through five years	1,991
After five years	82
Total	13,365	$ 16,499
Fair Value
Within one year	11,325
After one year through five years	1,992
After five years	82
Total	$ 13,399	$ 16,500