Pension and Other Postretirement Employee Benefit Plans - Plan Assets and Liabilities Measured At Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined benefit pension plans
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	$ 18,052	$ 19,603	$ 17,703
Total liabilities measured at fair value	(96)	(141)
Pension and OPEB plan assets and liabilities - supplemental information
Unfunded commitments to purchase limited partnership investments for the plan	521	605
Excluded amount of receivables for investments sold and dividends and interest receivables	340	377
Excluded amount of payables for investments purchased	479	561
Excluded amount of other liabilities	24	26
Defined benefit pension plans | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	0	0
Defined benefit pension plans | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	(96)	(141)
Defined benefit pension plans | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	0	0
Defined benefit pension plans | Certain investments measured at fair value ising 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	5,000	4,400
Defined benefit pension plans | Derivative payables
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	(96)	(141)
Defined benefit pension plans | Derivative payables | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	0	0
Defined benefit pension plans | Derivative payables | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	(96)	(141)
Defined benefit pension plans | Derivative payables | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total liabilities measured at fair value	0	0
Defined benefit pension plans | Total assets measured at fair value
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	13,295	15,580
Defined benefit pension plans | Total assets measured at fair value | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	8,044	11,284
Defined benefit pension plans | Total assets measured at fair value | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	4,941	3,986
Defined benefit pension plans | Total assets measured at fair value | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	310	310
Defined benefit pension plans | Cash and cash equivalents
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	344	174
Defined benefit pension plans | Cash and cash equivalents | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	343	173
Defined benefit pension plans | Cash and cash equivalents | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1	1
Defined benefit pension plans | Cash and cash equivalents | 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 | Equity securities
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	5,506	6,603
Defined benefit pension plans | Equity securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	5,342	6,407
Defined benefit pension plans | Equity securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	162	194
Defined benefit pension plans | 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 | Mutual funds
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	325
Defined benefit pension plans | Mutual funds | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	325
Defined benefit pension plans | Mutual funds | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	0
Defined benefit pension plans | Mutual funds | 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 | Collective investment funds
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	161	778
Defined benefit pension plans | Collective investment funds | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	161	778
Defined benefit pension plans | Collective investment funds | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	0
Defined benefit pension plans | Collective investment funds | 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 | Limited partnerships
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	40	60
Defined benefit pension plans | Limited partnerships | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	40	60
Defined benefit pension plans | Limited partnerships | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	0	0
Defined benefit pension plans | Limited partnerships | 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 | Corporate debt securities
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	3,543	2,648
Defined benefit pension plans | 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 | Corporate debt securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	3,540	2,644
Defined benefit pension plans | Corporate debt securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	3	4
Defined benefit pension plans | U.S. federal, state, local and non-U.S. government debt securities
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,934	1,880
Defined benefit pension plans | 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	1,191	1,096
Defined benefit pension plans | 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	743	784
Defined benefit pension plans | 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 | Mortgage-backed securities
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	357	194
Defined benefit pension plans | Mortgage-backed securities | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	82	92
Defined benefit pension plans | Mortgage-backed securities | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	272	100
Defined benefit pension plans | Mortgage-backed securities | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	3	2
Defined benefit pension plans | Derivative receivables
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	143	203
Defined benefit pension plans | Derivative receivables | 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 | Derivative receivables | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	143	203
Defined benefit pension plans | Derivative receivables | 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 | Other
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	1,267	2,715
Defined benefit pension plans | Other | Level 1
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	885	2,353
Defined benefit pension plans | Other | Level 2
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	80	60
Defined benefit pension plans | Other | Level 3
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	302	302
OPEB plans
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,633	2,757	$ 1,956
OPEB plans | COLI Policies
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	2,100	2,200
OPEB plans | Corporate Debt Securities, U.S. Federal, State, Local and Non-U.S. Government Debt Securities and Other and Cash and Cash Equivalents
Pension and OPEB plan assets and liabilities measured at fair value
Total assets measured at fair value	$ 561	$ 600