Pension and Other Postretirement Employee Benefit Plans - Plan Assets and Liabilities Measured At Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	$ 25,417	$ 23,366
Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	22,367	19,343
Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	7,031	6,739
Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	12,384	9,915
Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,952	2,689	$ 2,400
Defined benefit pension plans | U.S.
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	17,619	16,329	$ 14,521
Pension and OPEB plan assets and liabilities - supplemental information
Excluded amount of payables for investments sold and purchased	606	343
Defined benefit pension plans | U.S. | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	15,061	12,771
Defined benefit pension plans | U.S. | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	4,997	4,905
Defined benefit pension plans | U.S. | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	9,819	7,608
Defined benefit pension plans | U.S. | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	245	258
Defined benefit pension plans | U.S. | Certain investments measured at fair value using net asset value per share as practical expedient
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	3,200	3,900
Defined benefit pension plans | U.S. | Equity securities | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,355	2,264
Defined benefit pension plans | U.S. | Equity securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,353	2,259
Defined benefit pension plans | U.S. | Equity securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	3
Defined benefit pension plans | U.S. | Equity securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2	2
Defined benefit pension plans | U.S. | Corporate debt securities | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	7,425	6,476
Defined benefit pension plans | U.S. | Corporate debt securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	0
Defined benefit pension plans | U.S. | Corporate debt securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	7,414	6,474
Defined benefit pension plans | U.S. | Corporate debt securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	11	2
Defined benefit pension plans | U.S. | U.S. federal, state, local and non-U.S. government debt securities | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,755	2,017
Defined benefit pension plans | U.S. | U.S. federal, state, local and non-U.S. government debt securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,395	1,616
Defined benefit pension plans | U.S. | U.S. federal, state, local and non-U.S. government debt securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	360	401
Defined benefit pension plans | U.S. | U.S. federal, state, local and non-U.S. government debt securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	0
Defined benefit pension plans | U.S. | Mortgage-backed securities | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,676	997
Defined benefit pension plans | U.S. | Mortgage-backed securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	461	312
Defined benefit pension plans | U.S. | Mortgage-backed securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,184	681
Defined benefit pension plans | U.S. | Mortgage-backed securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	31	4
Defined benefit pension plans | U.S. | Other | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,850	1,017
Defined benefit pension plans | U.S. | Other | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	788	718
Defined benefit pension plans | U.S. | Other | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	861	49
Defined benefit pension plans | U.S. | Other | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	201	250
Defined benefit pension plans | U.S. | Participating Annuity Contracts | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	199	250
OPEB plans
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	7,798	7,037
OPEB plans | Total fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	7,306	6,572
OPEB plans | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,034	1,834
OPEB plans | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,565	2,307
OPEB plans | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,707	2,431
OPEB plans | Certain investments measured at fair value using net asset value per share as practical expedient
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	487	465
OPEB plans | COLI Policies | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	$ 2,700	$ 2,400