Debt Investment Maturities (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Cost Basis
Due in one year or less	$ 19,448
Due after one year through five years	103,950
Due after five years through 10 years	14,143
Due after 10 years	922
Total	138,463
Estimated Fair Value
Due in one year or less	19,447
Due after one year through five years	103,341
Due after five years through 10 years	14,083
Due after 10 years	951
Total	$ 137,822