Fair Value Disclosures - Fair Value of Financial Assets and Financial Liabilities (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Cash and cash equivalents | $ 5,978 | $ 6,894 |
| Money market valuation per share floor | $ 1.00 | |
| Money Market Funds [Member] | ||
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Cash and cash equivalents | $ 204 | 163 |
| Consolidated Variable Interest Entities [Member] | ||
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Cash and cash equivalents | 103 | 144 |
| Consolidated Variable Interest Entities [Member] | Money Market Funds [Member] | ||
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ||
| Cash and cash equivalents | $ 13 | $ 14 |