Accumulated Other Comprehensive Income - Summary of Changes in AOCI by Component (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ 7,866	$ 7,803	$ 8,830	$ 7,866	$ 9,109
Other comprehensive income (loss) before reclassifications				(318)	(263)
Amounts reclassified from AOCI				14	51
Net other comprehensive income (loss)		(202)	2	(304)	(212)
End of period		7,617	9,090	7,617	9,090
ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard	(161)			161
ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard					(3)
CTA
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	(2,868)			(2,868)	(2,545)
Other comprehensive income (loss) before reclassifications				(272)	(314)
Amounts reclassified from AOCI				0	0
Net other comprehensive income (loss)				(272)	(314)
End of period		(3,131)	(2,859)	(3,131)	(2,859)
CTA | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				(9)
CTA | ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard					0
Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	(954)			(954)	(987)
Other comprehensive income (loss) before reclassifications				20	50
Amounts reclassified from AOCI				17	44
Net other comprehensive income (loss)				37	94
End of period		(1,086)	(893)	(1,086)	(893)
Pension and OPEB Plans | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				169
Pension and OPEB Plans | ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard					0
Hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	(1)			(1)	(10)
Other comprehensive income (loss) before reclassifications				(66)	1
Amounts reclassified from AOCI				(3)	7
Net other comprehensive income (loss)				(69)	8
End of period		(71)	(2)	(71)	(2)
Hedging activities | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				1
Hedging activities | ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard					0
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period					3
Other comprehensive income (loss) before reclassifications					0
Amounts reclassified from AOCI					0
Net other comprehensive income (loss)					0
End of period			0		0
Available-for-sale securities | ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard					(3)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ (3,823)			(3,823)	(3,539)
End of period		$ (4,288)	$ (3,754)	$ (4,288)	$ (3,754)