Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,493	$ 10,033	$ 11,060	$ 10,892
Pension Plan | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	24	27
Pension Plan | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,864	4,925
Pension Plan | Investment Grade Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	211	109
Pension Plan | High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	7	7
Pension Plan | High Yield Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39	388
Pension Plan | Emerging Market Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0	1
Pension Plan | U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	169	150
Pension Plan | Non-U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	467	500
Pension Plan | Emerging Market Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92	75
Pension Plan | Private Equity | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	322	347
Pension Plan | Private Debt | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	483	452
Pension Plan | Market Neutral Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	433	746
Pension Plan | Directional Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	443	512
Pension Plan | Real Estate | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	635	821
Pension Plan | Multi-Asset Strategies | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	449	763
Pension Plan | Cash Equivalents and Short-term Investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	121	62
Pension Plan | Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,646	5,108
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,636	2,326
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,988	2,758
Pension Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	24
Pension Plan | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,864	4,925
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,025	1,851
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	828	458
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,197	1,393
Pension Plan | Fair Value, Measurements, Recurring | Investment Grade Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	237	284
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	232	277
Pension Plan | Fair Value, Measurements, Recurring | High Yield Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	7
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	287	332
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	203	151
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	84	181
Pension Plan | Fair Value, Measurements, Recurring | Emerging Market Bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	760	768
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	756	764
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	2
Pension Plan | Fair Value, Measurements, Recurring | U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	2
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	592	601
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	592	601
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Non-U.S. Stocks | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	15
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Private Debt | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	15
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	257	342
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	257	342
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Multi-Asset Strategies | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Repurchase Agreements
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39
Pension Plan | Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Pension Plan | Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39
Pension Plan | Fair Value, Measurements, Recurring | Repurchase Agreements | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Pension Plan | Fair Value, Measurements, Recurring | Cash Equivalents and Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	433	910
Pension Plan | Fair Value, Measurements, Recurring | Cash Equivalents and Short-term Investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
Pension Plan | Fair Value, Measurements, Recurring | Cash Equivalents and Short-term Investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	433	907
Pension Plan | Fair Value, Measurements, Recurring | Cash Equivalents and Short-term Investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plan | Fair Value, Measurements, Recurring | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5
Liabilities	(17)
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7
Liabilities	1
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		(2)
Liabilities	(18)
Pension Plan | Fair Value, Measurements, Recurring | Derivatives | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Liabilities	0
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	18	$ 23	$ 53
Other Postretirement Benefits Plan | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	18
Other Postretirement Benefits Plan | Investment Grade Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | High Yield Bonds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Non-U.S. Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Emerging Market Stocks | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Private Equity | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	6
Other Postretirement Benefits Plan | Private Debt | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Other Postretirement Benefits Plan | Market Neutral Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Directional Hedge Funds | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Real Estate | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Multi-Asset Strategies | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Cash Equivalents and Short-term Investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	11
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 13	$ 18