Pension and Other Postretirement Benefits - Benefit Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,550	$ 2,832	$ 2,487
Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	34
Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,229	1,332
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	333	373	$ 351
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
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	43
Cash and equivalents | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	34
Cash and equivalents | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	9
Cash and equivalents | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and equivalents | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2
Cash and equivalents | Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2
Cash and equivalents | Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Cash and equivalents | Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Fixed income securities, government securities | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	371	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	371	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	853	943
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	853	943
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
Collective trust funds | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,257	1,424
Collective trust funds | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Collective trust funds | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Collective trust funds | Pension | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Partnerships/private equity interests | Pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36	41
Partnerships/private equity interests | Pension | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Partnerships/private equity interests | Pension | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
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	9
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	9
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	333	371
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