Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance	$ 68	$ 48		$ 68	$ 48
Income tax adjustment	$ 180	$ 88	$ (3,218)	$ 180	$ (3,130)
Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			465
Pension and Other Postretirement Liabilities | Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			285
Fuel | Designated as Hedging Instrument | Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			$ 180