Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance	$ 68	$ 63
Income tax adjustment from AOCI	$ 180
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