Pension and Postretirement Benefit Plans - Pension Asset Fair Values (Details) - Funded - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Qualified and Non-qualified Pension Benefits | United States
Benefit Plan Information
Total, before other items to reconcile	$ 16,557	$ 15,062
Other items to reconcile to fair value of plan assets	(458)	(259)
Fair value of plan assets	16,099	14,803	$ 15,686
Qualified and Non-qualified Pension Benefits | International
Benefit Plan Information
Total, before other items to reconcile	6,976	6,223
Other items to reconcile to fair value of plan assets	(53)	(53)
Fair value of plan assets	6,923	6,170	6,737
Qualified and Non-qualified Pension Benefits | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	5,612	4,988
Qualified and Non-qualified Pension Benefits | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	1,873	1,578
Qualified and Non-qualified Pension Benefits | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	5,584	4,464
Qualified and Non-qualified Pension Benefits | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	3,778	2,828
Qualified and Non-qualified Pension Benefits | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	540	527
Qualified and Non-qualified Pension Benefits | Global equity | United States
Benefit Plan Information
Total, before other items to reconcile	3,577	2,975
Qualified and Non-qualified Pension Benefits | Global equity | International
Benefit Plan Information
Total, before other items to reconcile	2,312	2,014
Qualified and Non-qualified Pension Benefits | Global equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	3,160	2,603
Qualified and Non-qualified Pension Benefits | Global equity | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	1,395	961
Qualified and Non-qualified Pension Benefits | Global equity | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	894	1,032
Qualified and Non-qualified Pension Benefits | Global equity | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	5
Qualified and Non-qualified Pension Benefits | U.S. equities | United States
Benefit Plan Information
Total, before other items to reconcile	1,575	1,369
Qualified and Non-qualified Pension Benefits | U.S. equities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,575	1,369
Qualified and Non-qualified Pension Benefits | Non-U.S. equities | United States
Benefit Plan Information
Total, before other items to reconcile	1,585	1,234
Qualified and Non-qualified Pension Benefits | Non-U.S. equities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,585	1,234
Qualified and Non-qualified Pension Benefits | Growth equities | International
Benefit Plan Information
Total, before other items to reconcile	1,434	708
Qualified and Non-qualified Pension Benefits | Growth equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	638	460
Qualified and Non-qualified Pension Benefits | Growth equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	796	248
Qualified and Non-qualified Pension Benefits | Value equities | International
Benefit Plan Information
Total, before other items to reconcile	706	488
Qualified and Non-qualified Pension Benefits | Value equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	696	446
Qualified and Non-qualified Pension Benefits | Value equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	10	42
Qualified and Non-qualified Pension Benefits | Core equities | International
Benefit Plan Information
Total, before other items to reconcile	154	802
Qualified and Non-qualified Pension Benefits | Core equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	61	55
Qualified and Non-qualified Pension Benefits | Core equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	88	742
Qualified and Non-qualified Pension Benefits | Core equities | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	5
Qualified and Non-qualified Pension Benefits | Equities valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	18	16
Qualified and Non-qualified Pension Benefits | Index and long/short equity | United States
Benefit Plan Information
Total, before other items to reconcile	417	372
Qualified and Non-qualified Pension Benefits | Fixed income | United States
Benefit Plan Information
Total, before other items to reconcile	7,814	6,256
Qualified and Non-qualified Pension Benefits | Fixed income | International
Benefit Plan Information
Total, before other items to reconcile	3,475	3,180
Qualified and Non-qualified Pension Benefits | Fixed income | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	2,351	1,900
Qualified and Non-qualified Pension Benefits | Fixed income | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	378	540
Qualified and Non-qualified Pension Benefits | Fixed income | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	5,463	4,347
Qualified and Non-qualified Pension Benefits | Fixed income | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	2,635	1,665
Qualified and Non-qualified Pension Benefits | Fixed income | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	13	14
Qualified and Non-qualified Pension Benefits | U.S. government securities | United States
Benefit Plan Information
Total, before other items to reconcile	3,262	2,621
Qualified and Non-qualified Pension Benefits | U.S. government securities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	2,346	1,889
Qualified and Non-qualified Pension Benefits | U.S. government securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	916	732
Qualified and Non-qualified Pension Benefits | Domestic government debt | International
Benefit Plan Information
Total, before other items to reconcile	790	690
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	353	334
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	433	351
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	4	5
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | United States
Benefit Plan Information
Total, before other items to reconcile	61	44
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | International
Benefit Plan Information
Total, before other items to reconcile	625	471
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	22	150
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	61	44
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	603	321
Qualified and Non-qualified Pension Benefits | Preferred and convertible securities | United States
Benefit Plan Information
Total, before other items to reconcile	52	44
Qualified and Non-qualified Pension Benefits | Preferred and convertible securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	52	44
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | United States
Benefit Plan Information
Total, before other items to reconcile	3,576	2,950
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	10	9
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	3,566	2,941
Qualified and Non-qualified Pension Benefits | Non-U.S. corporate bonds | United States
Benefit Plan Information
Total, before other items to reconcile	759	475
Qualified and Non-qualified Pension Benefits | Non-U.S. corporate bonds | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	759	475
Qualified and Non-qualified Pension Benefits | Corporate debt securities | International
Benefit Plan Information
Total, before other items to reconcile	1,611	1,058
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	3	56
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	1,599	993
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	9	9
Qualified and Non-qualified Pension Benefits | Other | United States
Benefit Plan Information
Total, before other items to reconcile		9
Qualified and Non-qualified Pension Benefits | Derivative instruments | United States
Benefit Plan Information
Total, before other items to reconcile	104	113
Qualified and Non-qualified Pension Benefits | Derivative instruments | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	(5)	2
Qualified and Non-qualified Pension Benefits | Derivative instruments | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	109	111
Qualified and Non-qualified Pension Benefits | Fixed income securities valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	449	961
Qualified and Non-qualified Pension Benefits | Private equity | United States
Benefit Plan Information
Total, before other items to reconcile	1,945	2,109
Qualified and Non-qualified Pension Benefits | Private equity | International
Benefit Plan Information
Total, before other items to reconcile	338	210
Qualified and Non-qualified Pension Benefits | Private equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	80	45
Qualified and Non-qualified Pension Benefits | Private equity | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	6	5
Qualified and Non-qualified Pension Benefits | Private equity | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	207	75
Qualified and Non-qualified Pension Benefits | Private equity | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	4	4
Qualified and Non-qualified Pension Benefits | Growth equity | United States
Benefit Plan Information
Total, before other items to reconcile	80	45
Qualified and Non-qualified Pension Benefits | Growth equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	80	45
Qualified and Non-qualified Pension Benefits | Real estate | International
Benefit Plan Information
Total, before other items to reconcile	217	84
Qualified and Non-qualified Pension Benefits | Real estate | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	6	5
Qualified and Non-qualified Pension Benefits | Real estate | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	207	75
Qualified and Non-qualified Pension Benefits | Real estate | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	4	4
Qualified and Non-qualified Pension Benefits | Real estate valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	36	37
Qualified and Non-qualified Pension Benefits | Partnership investments | United States
Benefit Plan Information
Total, before other items to reconcile	1,865	2,064
Qualified and Non-qualified Pension Benefits | Partnership investments | International
Benefit Plan Information
Total, before other items to reconcile	85	89
Qualified and Non-qualified Pension Benefits | Absolute return | United States
Benefit Plan Information
Total, before other items to reconcile	2,717	2,437
Qualified and Non-qualified Pension Benefits | Absolute return | International
Benefit Plan Information
Total, before other items to reconcile	717	725
Qualified and Non-qualified Pension Benefits | Absolute return | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1	28
Qualified and Non-qualified Pension Benefits | Absolute return | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile		1
Qualified and Non-qualified Pension Benefits | Absolute return | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	117	114
Qualified and Non-qualified Pension Benefits | Absolute return | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	3	34
Qualified and Non-qualified Pension Benefits | Absolute return | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	518	504
Qualified and Non-qualified Pension Benefits | Insurance | International
Benefit Plan Information
Total, before other items to reconcile	513	496
Qualified and Non-qualified Pension Benefits | Insurance | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	513	496
Qualified and Non-qualified Pension Benefits | Hedge fund/fund of funds | United States
Benefit Plan Information
Total, before other items to reconcile	2,010	1,866
Qualified and Non-qualified Pension Benefits | Fixed income and other | United States
Benefit Plan Information
Total, before other items to reconcile	118	142
Qualified and Non-qualified Pension Benefits | Fixed income and other | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1	28
Qualified and Non-qualified Pension Benefits | Fixed income and other | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	117	114
Qualified and Non-qualified Pension Benefits | Partnership investments - absolute return | United States
Benefit Plan Information
Total, before other items to reconcile	589	429
Qualified and Non-qualified Pension Benefits | Derivatives - absolute return | International
Benefit Plan Information
Total, before other items to reconcile	3	2
Qualified and Non-qualified Pension Benefits | Derivatives - absolute return | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile		1
Qualified and Non-qualified Pension Benefits | Derivatives - absolute return | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	3	1
Qualified and Non-qualified Pension Benefits | Other | International
Benefit Plan Information
Total, before other items to reconcile	5	41
Qualified and Non-qualified Pension Benefits | Other | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile		33
Qualified and Non-qualified Pension Benefits | Other | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	8
Qualified and Non-qualified Pension Benefits | Other valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	1
Qualified and Non-qualified Pension Benefits | Hedge funds | International
Benefit Plan Information
Total, before other items to reconcile	195	186
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | United States
Benefit Plan Information
Total, before other items to reconcile	504	1,285
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | International
Benefit Plan Information
Total, before other items to reconcile	134	94
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	20	412
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	94	71
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	4	3
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	39	22
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | United States
Benefit Plan Information
Total, before other items to reconcile	25	416
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | International
Benefit Plan Information
Total, before other items to reconcile	133	93
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	20	412
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	94	71
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	5	4
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	39	22
Qualified and Non-qualified Pension Benefits | Repurchase Agreements And Derivative Margin Activity | United States
Benefit Plan Information
Total, before other items to reconcile	(1)	(1)
Qualified and Non-qualified Pension Benefits | Repurchase Agreements And Derivative Margin Activity | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	(1)	(1)
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents valued at net asset value | United States
Benefit Plan Information
Total, before other items to reconcile	480	870
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	1	1
Postretirement Benefits
Benefit Plan Information
Total, before other items to reconcile	1,361	1,272
Other items to reconcile to fair value of plan assets	(23)	(12)
Fair value of plan assets	$ 1,338	1,260	$ 1,397
Percentage of plan assets within 401h account	50.00%
Postretirement Benefits | Level 1
Benefit Plan Information
Total, before other items to reconcile	$ 587	576
Postretirement Benefits | Level 2
Benefit Plan Information
Total, before other items to reconcile	508	426
Postretirement Benefits | Global equity
Benefit Plan Information
Total, before other items to reconcile	447	448
Postretirement Benefits | Global equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	414	414
Postretirement Benefits | U.S. equities
Benefit Plan Information
Total, before other items to reconcile	337	356
Postretirement Benefits | U.S. equities | Level 1
Benefit Plan Information
Total, before other items to reconcile	337	356
Postretirement Benefits | Non-U.S. equities
Benefit Plan Information
Total, before other items to reconcile	77	58
Postretirement Benefits | Non-U.S. equities | Level 1
Benefit Plan Information
Total, before other items to reconcile	77	58
Postretirement Benefits | Index and long/short equity
Benefit Plan Information
Total, before other items to reconcile	33	34
Postretirement Benefits | Fixed income
Benefit Plan Information
Total, before other items to reconcile	638	528
Postretirement Benefits | Fixed income | Level 1
Benefit Plan Information
Total, before other items to reconcile	136	112
Postretirement Benefits | Fixed income | Level 2
Benefit Plan Information
Total, before other items to reconcile	502	416
Postretirement Benefits | U.S. government securities
Benefit Plan Information
Total, before other items to reconcile	378	325
Postretirement Benefits | U.S. government securities | Level 1
Benefit Plan Information
Total, before other items to reconcile	136	112
Postretirement Benefits | U.S. government securities | Level 2
Benefit Plan Information
Total, before other items to reconcile	242	213
Postretirement Benefits | Foreign government agency securities
Benefit Plan Information
Total, before other items to reconcile	6	4
Postretirement Benefits | Foreign government agency securities | Level 2
Benefit Plan Information
Total, before other items to reconcile	6	4
Postretirement Benefits | U.S. corporate bonds
Benefit Plan Information
Total, before other items to reconcile	203	162
Postretirement Benefits | U.S. corporate bonds | Level 2
Benefit Plan Information
Total, before other items to reconcile	203	162
Postretirement Benefits | Non-U.S. corporate bonds
Benefit Plan Information
Total, before other items to reconcile	46	32
Postretirement Benefits | Non-U.S. corporate bonds | Level 2
Benefit Plan Information
Total, before other items to reconcile	46	32
Postretirement Benefits | Derivative instruments
Benefit Plan Information
Total, before other items to reconcile	5	5
Postretirement Benefits | Derivative instruments | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	5
Postretirement Benefits | Private equity
Benefit Plan Information
Total, before other items to reconcile	96	103
Postretirement Benefits | Private equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	4	2
Postretirement Benefits | Growth equity
Benefit Plan Information
Total, before other items to reconcile	4	2
Postretirement Benefits | Growth equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	4	2
Postretirement Benefits | Partnership investments
Benefit Plan Information
Total, before other items to reconcile	92	101
Postretirement Benefits | Absolute return
Benefit Plan Information
Total, before other items to reconcile	124	104
Postretirement Benefits | Absolute return | Level 1
Benefit Plan Information
Total, before other items to reconcile		1
Postretirement Benefits | Absolute return | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	5
Postretirement Benefits | Hedge fund/fund of funds
Benefit Plan Information
Total, before other items to reconcile	92	80
Postretirement Benefits | Fixed income and other
Benefit Plan Information
Total, before other items to reconcile	5	6
Postretirement Benefits | Fixed income and other | Level 1
Benefit Plan Information
Total, before other items to reconcile		1
Postretirement Benefits | Fixed income and other | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	5
Postretirement Benefits | Partnership investments - absolute return
Benefit Plan Information
Total, before other items to reconcile	27	18
Postretirement Benefits | Cash and Cash Equivalent
Benefit Plan Information
Total, before other items to reconcile	56	89
Postretirement Benefits | Cash and Cash Equivalent | Level 1
Benefit Plan Information
Total, before other items to reconcile	33	47
Postretirement Benefits | Cash and Cash Equivalent | Level 2
Benefit Plan Information
Total, before other items to reconcile	1	5
Postretirement Benefits | Cash and cash equivalents
Benefit Plan Information
Total, before other items to reconcile	34	52
Postretirement Benefits | Cash and cash equivalents | Level 1
Benefit Plan Information
Total, before other items to reconcile	33	47
Postretirement Benefits | Cash and cash equivalents | Level 2
Benefit Plan Information
Total, before other items to reconcile	1	5
Postretirement Benefits | Cash and cash equivalents valued at net asset value
Benefit Plan Information
Total, before other items to reconcile	$ 22	$ 37