Income Taxes (Narrative) (Detail) $ in Millions |
3 Months Ended |
|---|---|
|
Mar. 31, 2018
USD ($)
| |
| Income Taxes [Line Items] | |
| Bonus Depreciation For Tax Purposes | 50.00% |
| Minimum [Member] | |
| Income Taxes [Line Items] | |
| Decrease in Unrecognized Tax Benefits is Reasonably Possible | $ 80 |
| Maximum [Member] | |
| Income Taxes [Line Items] | |
| Decrease in Unrecognized Tax Benefits is Reasonably Possible | $ 180 |