Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, net of tax		$ (734)	$ 8,659	$ (831)
Changes in value, net of tax		(83)		(103)
Amounts reclassified to earnings, net of tax		19		136
Total other comprehensive income (loss), net	$ 20	(64)	5	33
Ending Balance, net of tax	8,970	(798)	8,970	(798)
Pension and Other Postretirement Liabilities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax	(860)	(385)	(854)	(363)
Changes in value, before tax	(9)	(124)	(42)	(157)
Amounts reclassified to earnings, before tax	14	6	41	17
Net change, before tax	5	(118)	(1)	(140)
Ending Balance, before tax	(855)	(503)	(855)	(503)
Beginning Balance, deferred taxes	26	(146)	24	(154)
Changes in value, deferred taxes	3	45	15	57
Amounts reclassified to earnings, deferred taxes	(5)	(2)	(15)	(6)
Net change, deferred taxes	(2)	43		51
Ending Balance, deferred taxes	24	(103)	24	(103)
Derivative Contracts | Fuel
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax		(41)	(2)	(215)
Changes in value, before tax		(6)		(5)
Amounts reclassified to earnings, before tax		24	2	197
Net change, before tax		18	2	192
Ending Balance, before tax		(23)		(23)
Beginning Balance, deferred taxes		(165)	1	(102)
Changes in value, deferred taxes		2		2
Amounts reclassified to earnings, deferred taxes		(8)	(1)	(71)
Net change, deferred taxes		(6)	(1)	(69)
Ending Balance, deferred taxes		(171)		(171)
Investments and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax	(16)	3	1	3
Changes in value, before tax	26	1	9	1
Amounts reclassified to earnings, before tax		(2)		(2)
Net change, before tax	26	(1)	9	(1)
Ending Balance, before tax	10	2	10	2
Beginning Balance, deferred taxes	6		1
Changes in value, deferred taxes	(9)	(1)	(3)	(1)
Amounts reclassified to earnings, deferred taxes		$ 1	(1)	$ 1
Net change, deferred taxes	(9)		(4)
Ending Balance, deferred taxes	(3)		(3)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, net of tax	(844)		(829)
Changes in value, net of tax	11		(21)
Amounts reclassified to earnings, net of tax	9		26
Total other comprehensive income (loss), net	20		5
Ending Balance, net of tax	$ (824)		$ (824)