Retirement Plans (Fair Value of Plan Assets By Investment Category Within The Fair Value Hierarchy - Other Post-retirement Benefits) (Details) - Other  Post-retirement Benefits - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure Line Items
Total pension plan assets		$ 705	$ 644	$ 570
Fair Value
Compensation And Retirement Disclosure Line Items
Total pension plan assets		697	635
Fair Value | Cash and equivalents
Compensation And Retirement Disclosure Line Items
Total pension plan assets		16	18
Fair Value | Equity securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		97	92
Fair Value | Fixed-income securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		134	122
Fair Value | Equity funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		320	288
Fair Value | Fixed-income funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		128	113
Fair Value | Real estate funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		2	2
Quoted Prices in Active Markets for Identical Assets (Level 1)
Compensation And Retirement Disclosure Line Items
Total pension plan assets		99	94
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and equivalents
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		97	92
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed-income securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed-income funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		2	2
Significant Other Observable Inputs (Level 2)
Compensation And Retirement Disclosure Line Items
Total pension plan assets		598	541
Significant Other Observable Inputs (Level 2) | Cash and equivalents
Compensation And Retirement Disclosure Line Items
Total pension plan assets		16	18
Significant Other Observable Inputs (Level 2) | Equity securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Significant Other Observable Inputs (Level 2) | Fixed-income securities
Compensation And Retirement Disclosure Line Items
Total pension plan assets		134	122
Significant Other Observable Inputs (Level 2) | Equity funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		320	288
Significant Other Observable Inputs (Level 2) | Fixed-income funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		128	113
Significant Other Observable Inputs (Level 2) | Real estate funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets		0	0
Plan assets measured using NAV as a practical expedient | Real estate funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets	[1]	5	5
Plan assets measured using NAV as a practical expedient | Hedge funds
Compensation And Retirement Disclosure Line Items
Total pension plan assets	[1]	$ 3	$ 4

[1] Investments measured at fair value using NAV as a practical expedient are not classified in the fair value hierarchy. The fair value amounts presented in this table for these investments are included to permit reconciliation of the fair value hierarchy to the total plan assets.