Investments - Summary of Cost and Carrying Value of Trading Investments (Detail) - USD ($) $ in Millions |
Jun. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | $ 661 | $ 589 |
| Trading investments, Carrying Value | 719 | 625 |
| Deferred Compensation Plan Fund [Member] | ||
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | 34 | 41 |
| Trading investments, Carrying Value | 52 | 59 |
| Equity Securities/Multi-Asset Mutual Funds [Member] | ||
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | 374 | 290 |
| Trading investments, Carrying Value | 408 | 308 |
| Corporate Debt [Member] | ||
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | 174 | 128 |
| Trading investments, Carrying Value | 179 | 128 |
| Government Debt [Member] | ||
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | 30 | 60 |
| Trading investments, Carrying Value | 30 | 60 |
| Asset/Mortgage Backed Debt [Member] | ||
| Schedule of Investments [Line Items] | ||
| Trading investments, Cost | 49 | 70 |
| Trading investments, Carrying Value | $ 50 | $ 70 |