Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Within one year	$ 22,786
After one year through five years	1,364
After five years through ten years	46
After ten years	20
Amortized Cost	24,216	$ 29,685
Fair Value
Within one year	22,741
After one year through five years	1,358
After five years through ten years	45
After ten years	20
Available-for-sale debt securities	$ 24,164	$ 29,587