Pension and Other Postretirement Benefits - Benefit Plan Assets by Category and Valuation Methodology (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,844	$ 2,550	$ 2,832
Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	27
Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,329	1,229
Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	374	333	$ 373
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and equivalents | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	28
Cash and equivalents | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	27
Cash and equivalents | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Cash and equivalents | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, government securities | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	355	371
Fixed income securities, government securities | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, government securities | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	355	371
Fixed income securities, government securities | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, corporate debt securities | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	969	853
Fixed income securities, corporate debt securities | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, corporate debt securities | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	969	853
Fixed income securities, corporate debt securities | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, investment contracts with insurance companies | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Fixed income securities, investment contracts with insurance companies | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, investment contracts with insurance companies | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities, investment contracts with insurance companies | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Commingled funds, collective trust funds | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,460	1,257
Commingled funds, collective trust funds | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Commingled funds, collective trust funds | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Commingled funds, collective trust funds | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Commingled funds, partnerships/private equity interests | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	36
Commingled funds, partnerships/private equity interests | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Commingled funds, partnerships/private equity interests | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Commingled funds, partnerships/private equity interests | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Other | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	4
Other | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	4
Other | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Life insurance policies | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	374	333
Life insurance policies | Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Life insurance policies | Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Life insurance policies | Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets