Income Taxes - 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	Dec. 31, 2014
Income Taxes [Line Items]
Federal statutory income tax rate					35.00%
Deferred income tax expense				$ 1,648	$ (3,177)	$ 13
Alternative minimum tax ("AMT") credit carryforwards, net of valuation allowance	$ 108			108
Alternative minimum tax ("AMT") credit carryforwards, valuation allowance	8			8
Income tax adjustments	180	$ 88	$ (3,218)	180	(3,130)
Valuation allowance	68	48		68	48
Unrecognized tax benefits	74	24		74	24	$ 9
Unrecognized tax benefits that would impact effective tax rate if recognized	20			$ 20
Minimum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years				5 years
Maximum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years				20 years
2026
Income Taxes [Line Items]
Net operating loss expiration amount	500			$ 500
2027
Income Taxes [Line Items]
Net operating loss expiration amount	1,400			1,400
Thereafter
Income Taxes [Line Items]
Net operating loss expiration amount	2,500			2,500
Fuel | Designated as Hedging Instrument
Income Taxes [Line Items]
Deferred income tax expense				180	180
State
Income Taxes [Line Items]
NOL carry forwards	56			56
NOL carry forwards, net of valuation allowance	36			36
Valuation allowance		$ 68			$ 68
Federal
Income Taxes [Line Items]
NOL carry forwards	$ 4,400			$ 4,400