Cash, Cash Equivalents, and Marketable Securities - Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) $ in Millions	Dec. 31, 2016 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 19,656
Estimated Fair Value	19,647
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	16,532
Estimated Fair Value	16,531
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	2,766
Estimated Fair Value	2,760
Due after five years through ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	203
Estimated Fair Value	202
Due after ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	155
Estimated Fair Value	$ 154