Debt Investment Maturities (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Cost Basis
Due in one year or less	$ 16,698
Due after one year through five years	94,607
Due after five years through 10 years	16,427
Due after 10 years	949
Total	128,681
Estimated Fair Value
Due in one year or less	16,670
Due after one year through five years	93,722
Due after five years through 10 years	16,133
Due after 10 years	956
Total	$ 127,481