Pension and Postretirement Benefit Plans - Pension Asset Fair Values (Details) - Funded - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Qualified and Non-qualified Pension Benefits | United States
Benefit Plan Information
Total, before other items to reconcile	$ 15,062	$ 15,939
Other items to reconcile to fair value of plan assets	(259)	(253)
Fair value of plan assets	14,803	15,686	$ 14,081
Qualified and Non-qualified Pension Benefits | International
Benefit Plan Information
Total, before other items to reconcile	6,223	6,758
Other items to reconcile to fair value of plan assets	(53)	(21)
Fair value of plan assets	6,170	6,737	5,617
Qualified and Non-qualified Pension Benefits | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	4,988	5,949
Qualified and Non-qualified Pension Benefits | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	1,578	2,014
Qualified and Non-qualified Pension Benefits | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	4,464	3,896
Qualified and Non-qualified Pension Benefits | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	2,828	2,898
Qualified and Non-qualified Pension Benefits | Level 3 | United States
Benefit Plan Information
Total, before other items to reconcile		(6)
Qualified and Non-qualified Pension Benefits | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	527	547
Qualified and Non-qualified Pension Benefits | Global equity | United States
Benefit Plan Information
Total, before other items to reconcile	2,975	3,517
Qualified and Non-qualified Pension Benefits | Global equity | International
Benefit Plan Information
Total, before other items to reconcile	2,014	2,465
Qualified and Non-qualified Pension Benefits | Global equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	2,603	3,095
Qualified and Non-qualified Pension Benefits | Global equity | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	961	1,318
Qualified and Non-qualified Pension Benefits | Global equity | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	1,032	1,122
Qualified and Non-qualified Pension Benefits | Global equity | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	4
Qualified and Non-qualified Pension Benefits | U.S. equities | United States
Benefit Plan Information
Total, before other items to reconcile	1,369	1,568
Qualified and Non-qualified Pension Benefits | U.S. equities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,369	1,568
Qualified and Non-qualified Pension Benefits | Non-U.S. equities | United States
Benefit Plan Information
Total, before other items to reconcile	1,234	1,527
Qualified and Non-qualified Pension Benefits | Non-U.S. equities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,234	1,527
Qualified and Non-qualified Pension Benefits | Growth equities | International
Benefit Plan Information
Total, before other items to reconcile	708	948
Qualified and Non-qualified Pension Benefits | Growth equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	460	659
Qualified and Non-qualified Pension Benefits | Growth equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	248	289
Qualified and Non-qualified Pension Benefits | Value equities | International
Benefit Plan Information
Total, before other items to reconcile	488	640
Qualified and Non-qualified Pension Benefits | Value equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	446	597
Qualified and Non-qualified Pension Benefits | Value equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	42	43
Qualified and Non-qualified Pension Benefits | Core equities | International
Benefit Plan Information
Total, before other items to reconcile	802	856
Qualified and Non-qualified Pension Benefits | Core equities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	55	62
Qualified and Non-qualified Pension Benefits | Core equities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	742	790
Qualified and Non-qualified Pension Benefits | Core equities | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	4
Qualified and Non-qualified Pension Benefits | Equities valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	16	21
Qualified and Non-qualified Pension Benefits | Index and long/short equity | United States
Benefit Plan Information
Total, before other items to reconcile	372	422
Qualified and Non-qualified Pension Benefits | Fixed income | United States
Benefit Plan Information
Total, before other items to reconcile	6,256	6,970
Qualified and Non-qualified Pension Benefits | Fixed income | International
Benefit Plan Information
Total, before other items to reconcile	3,180	3,249
Qualified and Non-qualified Pension Benefits | Fixed income | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,900	2,680
Qualified and Non-qualified Pension Benefits | Fixed income | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	540	593
Qualified and Non-qualified Pension Benefits | Fixed income | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	4,347	4,282
Qualified and Non-qualified Pension Benefits | Fixed income | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	1,665	1,666
Qualified and Non-qualified Pension Benefits | Fixed income | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	14	14
Qualified and Non-qualified Pension Benefits | U.S. government securities | United States
Benefit Plan Information
Total, before other items to reconcile	2,621	3,150
Qualified and Non-qualified Pension Benefits | U.S. government securities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	1,889	2,666
Qualified and Non-qualified Pension Benefits | U.S. government securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	732	484
Qualified and Non-qualified Pension Benefits | Domestic government debt | International
Benefit Plan Information
Total, before other items to reconcile	690	622
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	334	362
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	351	256
Qualified and Non-qualified Pension Benefits | Domestic government debt | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	5	4
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | United States
Benefit Plan Information
Total, before other items to reconcile	44	168
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | International
Benefit Plan Information
Total, before other items to reconcile	471	506
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	150	168
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	44	168
Qualified and Non-qualified Pension Benefits | Foreign government agency securities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	321	338
Qualified and Non-qualified Pension Benefits | Preferred and convertible securities | United States
Benefit Plan Information
Total, before other items to reconcile	44	6
Qualified and Non-qualified Pension Benefits | Preferred and convertible securities | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile		4
Qualified and Non-qualified Pension Benefits | Preferred and convertible securities | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	44	2
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | United States
Benefit Plan Information
Total, before other items to reconcile	2,950	2,914
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	9	10
Qualified and Non-qualified Pension Benefits | U.S. corporate bonds | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	2,941	2,904
Qualified and Non-qualified Pension Benefits | Non-U.S. corporate bonds | United States
Benefit Plan Information
Total, before other items to reconcile	475	614
Qualified and Non-qualified Pension Benefits | Non-U.S. corporate bonds | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	475	614
Qualified and Non-qualified Pension Benefits | Corporate debt securities | International
Benefit Plan Information
Total, before other items to reconcile	1,058	1,145
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	56	63
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	993	1,072
Qualified and Non-qualified Pension Benefits | Corporate debt securities | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	9	10
Qualified and Non-qualified Pension Benefits | Other | United States
Benefit Plan Information
Total, before other items to reconcile	9	8
Qualified and Non-qualified Pension Benefits | Derivative instruments | United States
Benefit Plan Information
Total, before other items to reconcile	113	110
Qualified and Non-qualified Pension Benefits | Derivative instruments | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	2
Qualified and Non-qualified Pension Benefits | Derivative instruments | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	111	110
Qualified and Non-qualified Pension Benefits | Fixed income securities valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	961	976
Qualified and Non-qualified Pension Benefits | Private equity | United States
Benefit Plan Information
Total, before other items to reconcile	2,109	2,089
Qualified and Non-qualified Pension Benefits | Private equity | International
Benefit Plan Information
Total, before other items to reconcile	210	217
Qualified and Non-qualified Pension Benefits | Private equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	45	34
Qualified and Non-qualified Pension Benefits | Private equity | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	5	36
Qualified and Non-qualified Pension Benefits | Private equity | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	75	74
Qualified and Non-qualified Pension Benefits | Private equity | Level 3 | United States
Benefit Plan Information
Total, before other items to reconcile		(7)
Qualified and Non-qualified Pension Benefits | Private equity | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	4	3
Qualified and Non-qualified Pension Benefits | Derivative instruments | United States
Benefit Plan Information
Total, before other items to reconcile		(7)
Qualified and Non-qualified Pension Benefits | Derivative instruments | Level 3 | United States
Benefit Plan Information
Total, before other items to reconcile		(7)
Qualified and Non-qualified Pension Benefits | Growth equity | United States
Benefit Plan Information
Total, before other items to reconcile	45	34
Qualified and Non-qualified Pension Benefits | Growth equity | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	45	34
Qualified and Non-qualified Pension Benefits | Real estate | International
Benefit Plan Information
Total, before other items to reconcile	84	113
Qualified and Non-qualified Pension Benefits | Real estate | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	5	36
Qualified and Non-qualified Pension Benefits | Real estate | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	75	74
Qualified and Non-qualified Pension Benefits | Real estate | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	4	3
Qualified and Non-qualified Pension Benefits | Real estate valued at net asset value | International
Benefit Plan Information
Total, before other items to reconcile	37	36
Qualified and Non-qualified Pension Benefits | Partnership investments | United States
Benefit Plan Information
Total, before other items to reconcile	2,064	2,062
Qualified and Non-qualified Pension Benefits | Partnership investments | International
Benefit Plan Information
Total, before other items to reconcile	89	68
Qualified and Non-qualified Pension Benefits | Absolute return | United States
Benefit Plan Information
Total, before other items to reconcile	2,437	2,339
Qualified and Non-qualified Pension Benefits | Absolute return | International
Benefit Plan Information
Total, before other items to reconcile	725	723
Qualified and Non-qualified Pension Benefits | Absolute return | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	28	31
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	114	102
Qualified and Non-qualified Pension Benefits | Absolute return | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	34	2
Qualified and Non-qualified Pension Benefits | Absolute return | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	504	526
Qualified and Non-qualified Pension Benefits | Insurance | International
Benefit Plan Information
Total, before other items to reconcile	496	519
Qualified and Non-qualified Pension Benefits | Insurance | Level 3 | International
Benefit Plan Information
Total, before other items to reconcile	496	519
Qualified and Non-qualified Pension Benefits | Hedge fund/fund of funds | United States
Benefit Plan Information
Total, before other items to reconcile	1,866	1,871
Qualified and Non-qualified Pension Benefits | Fixed income and other | United States
Benefit Plan Information
Total, before other items to reconcile	142	133
Qualified and Non-qualified Pension Benefits | Fixed income and other | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	28	31
Qualified and Non-qualified Pension Benefits | Fixed income and other | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	114	102
Qualified and Non-qualified Pension Benefits | Partnership investments - absolute return | United States
Benefit Plan Information
Total, before other items to reconcile	429	335
Qualified and Non-qualified Pension Benefits | Derivatives - absolute return | International
Benefit Plan Information
Total, before other items to reconcile	2	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	1	2
Qualified and Non-qualified Pension Benefits | Other | International
Benefit Plan Information
Total, before other items to reconcile	41	7
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	8	7
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	186	194
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | United States
Benefit Plan Information
Total, before other items to reconcile	1,285	1,024
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | International
Benefit Plan Information
Total, before other items to reconcile	94	104
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	412	109
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	71	67
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	3	(488)
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	22	34
Qualified and Non-qualified Pension Benefits | Cash and Cash Equivalent | Level 3 | United States
Benefit Plan Information
Total, before other items to reconcile		1
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | United States
Benefit Plan Information
Total, before other items to reconcile	416	124
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | International
Benefit Plan Information
Total, before other items to reconcile	93	101
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 1 | United States
Benefit Plan Information
Total, before other items to reconcile	412	109
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 1 | International
Benefit Plan Information
Total, before other items to reconcile	71	67
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 2 | United States
Benefit Plan Information
Total, before other items to reconcile	4	14
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 2 | International
Benefit Plan Information
Total, before other items to reconcile	22	34
Qualified and Non-qualified Pension Benefits | Cash and cash equivalents | Level 3 | United States
Benefit Plan Information
Total, before other items to reconcile		1
Qualified and Non-qualified Pension Benefits | Repurchase Agreements And Derivative Margin Activity | United States
Benefit Plan Information
Total, before other items to reconcile	(1)	(502)
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)	(502)
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	870	1,402
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	3
Postretirement Benefits
Benefit Plan Information
Total, before other items to reconcile	1,272	1,407
Other items to reconcile to fair value of plan assets	(12)	(10)
Fair value of plan assets	$ 1,260	1,397	$ 1,356
Percentage of plan assets within 401h account	50.00%
Postretirement Benefits | Level 1
Benefit Plan Information
Total, before other items to reconcile	$ 576	709
Postretirement Benefits | Level 2
Benefit Plan Information
Total, before other items to reconcile	426	397
Postretirement Benefits | Global equity
Benefit Plan Information
Total, before other items to reconcile	448	581
Postretirement Benefits | Global equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	414	536
Postretirement Benefits | U.S. equities
Benefit Plan Information
Total, before other items to reconcile	356	465
Postretirement Benefits | U.S. equities | Level 1
Benefit Plan Information
Total, before other items to reconcile	356	465
Postretirement Benefits | Non-U.S. equities
Benefit Plan Information
Total, before other items to reconcile	58	71
Postretirement Benefits | Non-U.S. equities | Level 1
Benefit Plan Information
Total, before other items to reconcile	58	71
Postretirement Benefits | Index and long/short equity
Benefit Plan Information
Total, before other items to reconcile	34	45
Postretirement Benefits | Fixed income
Benefit Plan Information
Total, before other items to reconcile	528	548
Postretirement Benefits | Fixed income | Level 1
Benefit Plan Information
Total, before other items to reconcile	112	136
Postretirement Benefits | Fixed income | Level 2
Benefit Plan Information
Total, before other items to reconcile	416	412
Postretirement Benefits | U.S. government securities
Benefit Plan Information
Total, before other items to reconcile	325	341
Postretirement Benefits | U.S. government securities | Level 1
Benefit Plan Information
Total, before other items to reconcile	112	136
Postretirement Benefits | U.S. government securities | Level 2
Benefit Plan Information
Total, before other items to reconcile	213	205
Postretirement Benefits | Foreign government agency securities
Benefit Plan Information
Total, before other items to reconcile	4	9
Postretirement Benefits | Foreign government agency securities | Level 2
Benefit Plan Information
Total, before other items to reconcile	4	9
Postretirement Benefits | U.S. corporate bonds
Benefit Plan Information
Total, before other items to reconcile	162	159
Postretirement Benefits | U.S. corporate bonds | Level 2
Benefit Plan Information
Total, before other items to reconcile	162	159
Postretirement Benefits | Non-U.S. corporate bonds
Benefit Plan Information
Total, before other items to reconcile	32	35
Postretirement Benefits | Non-U.S. corporate bonds | Level 2
Benefit Plan Information
Total, before other items to reconcile	32	35
Postretirement Benefits | Derivative instruments
Benefit Plan Information
Total, before other items to reconcile	5	4
Postretirement Benefits | Derivative instruments | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	4
Postretirement Benefits | Private equity
Benefit Plan Information
Total, before other items to reconcile	103	111
Postretirement Benefits | Private equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	2	2
Postretirement Benefits | Growth equity
Benefit Plan Information
Total, before other items to reconcile	2	2
Postretirement Benefits | Growth equity | Level 1
Benefit Plan Information
Total, before other items to reconcile	2	2
Postretirement Benefits | Partnership investments
Benefit Plan Information
Total, before other items to reconcile	101	109
Postretirement Benefits | Absolute return
Benefit Plan Information
Total, before other items to reconcile	104	95
Postretirement Benefits | Absolute return | Level 1
Benefit Plan Information
Total, before other items to reconcile	1	1
Postretirement Benefits | Absolute return | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	4
Postretirement Benefits | Hedge fund/fund of funds
Benefit Plan Information
Total, before other items to reconcile	80	76
Postretirement Benefits | Fixed income and other
Benefit Plan Information
Total, before other items to reconcile	6	5
Postretirement Benefits | Fixed income and other | Level 1
Benefit Plan Information
Total, before other items to reconcile	1	1
Postretirement Benefits | Fixed income and other | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	4
Postretirement Benefits | Partnership investments - absolute return
Benefit Plan Information
Total, before other items to reconcile	18	14
Postretirement Benefits | Cash and Cash Equivalent
Benefit Plan Information
Total, before other items to reconcile	89	72
Postretirement Benefits | Cash and Cash Equivalent | Level 1
Benefit Plan Information
Total, before other items to reconcile	47	34
Postretirement Benefits | Cash and Cash Equivalent | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	(19)
Postretirement Benefits | Cash and cash equivalents
Benefit Plan Information
Total, before other items to reconcile	52	35
Postretirement Benefits | Cash and cash equivalents | Level 1
Benefit Plan Information
Total, before other items to reconcile	47	34
Postretirement Benefits | Cash and cash equivalents | Level 2
Benefit Plan Information
Total, before other items to reconcile	5	1
Postretirement Benefits | Repurchase Agreements And Derivative Margin Activity
Benefit Plan Information
Total, before other items to reconcile		(20)
Postretirement Benefits | Repurchase Agreements And Derivative Margin Activity | Level 2
Benefit Plan Information
Total, before other items to reconcile		(20)
Postretirement Benefits | Cash and cash equivalents valued at net asset value
Benefit Plan Information
Total, before other items to reconcile	$ 37	$ 57