Changes in Accumulated Other Comprehensive Losses - Summary of Changed (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	$ 155,642	$ 149,319	$ 146,722
Components of Other Comprehensive Income (Loss):
Before Reclassifications	(1,855)	4	(256)
Reclassifications	409	603	510
Other Comprehensive Gain (Loss), Net of Tax	(1,446)	607	254
Balance at December 31	145,208	155,642	149,319
Income tax expense (benefit)	2,691	5,715	(48)
Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	(124)	(105)	(162)
Components of Other Comprehensive Income (Loss):
Before Reclassifications	(18)	(19)	57
Reclassifications	0	0	0
Other Comprehensive Gain (Loss), Net of Tax	(18)	(19)	57
Balance at December 31	(142)	(124)	(105)
Unrealized Holding Gains (Losses) on Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	(10)	(5)	(2)
Components of Other Comprehensive Income (Loss):
Before Reclassifications	2	(5)	(3)
Reclassifications	0	0	0
Other Comprehensive Gain (Loss), Net of Tax	2	(5)	(3)
Balance at December 31	(8)	(10)	(5)
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	(2)	(2)	(2)
Components of Other Comprehensive Income (Loss):
Before Reclassifications	(1)	0	0
Reclassifications	3	0	0
Other Comprehensive Gain (Loss), Net of Tax	2	0	0
Balance at December 31	0	(2)	(2)
Defined Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	(3,408)	(3,477)	(3,677)
Components of Other Comprehensive Income (Loss):
Before Reclassifications	(1,838)	28	(310)
Reclassifications	406	603	510
Other Comprehensive Gain (Loss), Net of Tax	(1,432)	631	200
Stranded Tax Reclassification to Retained Earnings		(562)
Balance at December 31	(4,840)	(3,408)	(3,477)
Defined Benefit Plans | Reclassification out of Accumulated Other Comprehensive Income
Components of Other Comprehensive Income (Loss):
Reclassification, before tax amount	523
Income tax expense (benefit)	117
Accumulated Other Comprehensive Loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at January 1	(3,544)	(3,589)	(3,843)
Components of Other Comprehensive Income (Loss):
Other Comprehensive Gain (Loss), Net of Tax	(1,446)	607	254
Stranded Tax Reclassification to Retained Earnings		(562)
Balance at December 31	$ (4,990)	$ (3,544)	$ (3,589)