Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, net of tax		$ (777)	$ 8,659	$ (831)
Changes in value, net of tax		17		(20)
Amounts reclassified to earnings, net of tax		26		117
Total other comprehensive income (loss), net	$ (8)	43	(15)	97
Ending Balance, net of tax	8,854	(734)	8,854	(734)
Pension and Other Postretirement Liabilities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax	(867)	(401)	(854)	(363)
Changes in value, before tax	(7)	10	(33)	(33)
Amounts reclassified to earnings, before tax	14	6	27	11
Net change, before tax	7	16	(6)	(22)
Ending Balance, before tax	(860)	(385)	(860)	(385)
Beginning Balance, deferred taxes	29	(140)	24	(154)
Changes in value, deferred taxes	2	(4)	12	12
Amounts reclassified to earnings, deferred taxes	(5)	(2)	(10)	(4)
Net change, deferred taxes	(3)	(6)	2	8
Ending Balance, deferred taxes	26	(146)	26	(146)
Derivative Contracts | Fuel
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax		(93)	(2)	(215)
Changes in value, before tax		17		1
Amounts reclassified to earnings, before tax		35	2	173
Net change, before tax		52	2	174
Ending Balance, before tax		(41)		(41)
Beginning Balance, deferred taxes		(146)	1	(102)
Changes in value, deferred taxes		(6)
Amounts reclassified to earnings, deferred taxes		(13)	(1)	(63)
Net change, deferred taxes		(19)	(1)	(63)
Ending Balance, deferred taxes		(165)		(165)
Investments and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, before tax	1	3	1	3
Changes in value, before tax	(17)		(17)
Net change, before tax	(17)		(17)
Ending Balance, before tax	(16)	$ 3	(16)	$ 3
Beginning Balance, deferred taxes	1		1
Changes in value, deferred taxes	6		6
Amounts reclassified to earnings, deferred taxes	(1)		(1)
Net change, deferred taxes	5		5
Ending Balance, deferred taxes	6		6
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, net of tax	(836)		(829)
Changes in value, net of tax	(16)		(32)
Amounts reclassified to earnings, net of tax	8		17
Total other comprehensive income (loss), net	(8)		(15)
Ending Balance, net of tax	$ (844)		$ (844)