Benefit Plans (Fair Value of Plan Assets Measured on a Recurring Basis) (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,420	$ 2,285	$ 2,025
Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,420	2,285
Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,664	1,472
Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	699	526
Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	956	936
Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	10
Fair Value Measured at Net Asset Value Per Share | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	756	813
Total fixed maturity securities trading | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	782	802
Total fixed maturity securities trading | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total fixed maturity securities trading | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	773	792
Total fixed maturity securities trading | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	10
Corporate and other bonds | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	652	597
Corporate and other bonds | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate and other bonds | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	643	587
Corporate and other bonds | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	10
States, municipalities and political subdivisions | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32	51
States, municipalities and political subdivisions | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32	51
States, municipalities and political subdivisions | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Asset-backed | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	98	154
Asset-backed | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Asset-backed | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	98	154
Asset-backed | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	803	586
Equity securities | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	666	458
Equity securities | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	137	128
Equity securities | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Short term investments | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	62
Short term investments | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	55
Short term investments | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38	7
Short term investments | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	9
Other assets | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	9
Other assets | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Total | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	13
Cash | Level 1 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	13
Cash | Level 2 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Level 3 | Fair Value, Recurring
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0