Pension Plans and Other Postretirement Benefits - Fair Value of Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,395	$ 5,663	$ 0
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,121	1,709
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,196	2,895
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	354	430
Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,671	5,034
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92	68
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	65	30
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	157	98
Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	592	511
Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,844	2,610
Debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	5
Debt securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,437	3,126
U.s. government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	592	511
U.s. government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	38
U.s. government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.s. government securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	623	549
States, municipalities and political subdivisions | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	157	147
States, municipalities and political subdivisions | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	157	147
U.S. corporate securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. corporate securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,849	1,671
U.S. corporate securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	5
U.S. corporate securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,850	1,676
Foreign securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	178	177
Foreign securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	178	177
Residential mortgage-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Residential mortgage-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	385	339
Residential mortgage-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Residential mortgage-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	385	339
Commercial mortgage-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Commercial mortgage-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89	70
Commercial mortgage-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Commercial mortgage-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89	70
Other asset-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other asset-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	150	162
Other asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other asset-backed securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	150	162
Redeemable preferred securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Redeemable preferred securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	6
Redeemable preferred securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Redeemable preferred securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	6
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,437	1,130
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,438	1,130
U.S. Domestic | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	931	744
U.S. Domestic | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
U.S. Domestic | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Domestic | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	932	744
International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	481	356
International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	481	356
Domestic real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25	30
Domestic real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Domestic real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Domestic real estate | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25	30
Other investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	286	255
Other investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	353	425
Other investments | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	639	680
Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	353	425
Real estate | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	353	425
Common/collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common/collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	288	253
Common/collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common/collective trusts | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	288	253
Derivatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Derivatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(2)	2
Derivatives | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Derivatives | Total Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(2)	2
Common/collective trusts, Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		109
Common/collective trusts, Equity Securities | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	137
Common/collective trusts, Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		144
Common/collective trusts, Debt Securities | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151
Private equity investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	540	465
Hedge fund investments | Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 184	$ 164