Accumulated Other Comprehensive Income - Summary of Changes in AOCI by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ 8,160	$ 7,803	$ 7,912	$ 7,866
Other comprehensive income (loss) before reclassifications			(162)	(318)
Amounts reclassified from AOCI			33	14
Net other comprehensive (loss) income	149	(202)	(129)	(304)
End of period	8,602	7,617	8,602	7,617
ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				(161)
CTA
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period			(2,954)	(2,868)
Other comprehensive income (loss) before reclassifications			(43)	(272)
Amounts reclassified from AOCI			0	0
Net other comprehensive (loss) income			(43)	(272)
End of period	(2,997)	(3,131)	(2,997)	(3,131)
CTA | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				9
Pension and OPEB Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period			(715)	(954)
Other comprehensive income (loss) before reclassifications			(9)	20
Amounts reclassified from AOCI			35	17
Net other comprehensive (loss) income			26	37
End of period	(689)	(1,086)	(689)	(1,086)
Pension and OPEB Plans | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				(169)
Hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period			(41)	(1)
Other comprehensive income (loss) before reclassifications			(110)	(66)
Amounts reclassified from AOCI			(2)	(3)
Net other comprehensive (loss) income			(112)	(69)
End of period	(153)	(71)	(153)	(71)
Hedging activities | ASU 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard				(1)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period			(3,710)	(3,823)
End of period	$ (3,839)	$ (4,288)	$ (3,839)	$ (4,288)