Income Taxes (Narrative) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Income Taxes [Line Items]
Bonus Depreciation For Tax Purposes	50.00%
Bonus Depreciation for Tax Purposes 2018	40.00%
Bonus Depreciation for Tax Purposes 2019	30.00%
Current ITC rate for qualified property	30.00%
2020 ITC rate for qualified property	26.00%
2021 ITC rate for qualified property	22.00%
Long production property [Member]
Income Taxes [Line Items]
Bonus Depreciation for Tax Purposes 2018	50.00%
Bonus Depreciation for Tax Purposes 2019	40.00%
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