Accumulated Other Comprehensive Income - Summary of Changes in AOCI by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ 7,912	$ 7,866
Other comprehensive income (loss) before reclassifications	(480)	(105)
Amounts reclassified from AOCI	13	7
Net other comprehensive income (loss)	(467)	(98)
End of period	7,730	7,593
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	(359)	(97)
Amounts reclassified from AOCI	0	0
Net other comprehensive income (loss)	(359)	(97)
End of period	(3,313)	(2,956)
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	10	7
Amounts reclassified from AOCI	12	7
Net other comprehensive income (loss)	22	14
End of period	(693)	(1,109)
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	(131)	(15)
Amounts reclassified from AOCI	1	0
Net other comprehensive income (loss)	(130)	(15)
End of period	(171)	(17)
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	$ (4,177)	$ (4,082)