Benefit Plans (Fair Value of Plan Assets Measured on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Other assets	$ 2,261	$ 2,193
Corporate and other bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	532	510
Corporate and other bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate and other bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	522	500
Corporate and other bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
States, municipalities and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62	63
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	62	63
States, municipalities and political subdivisions | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total asset-backed
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	180	179
Total asset-backed | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total asset-backed | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	180	179
Total asset-backed | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total fixed maturity securities trading
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	774	752
Total fixed maturity securities trading | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total fixed maturity securities trading | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	764	742
Total fixed maturity securities trading | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	527	468
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	405	363
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122	105
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Short term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34	46
Short term investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	11
Short term investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	35
Short term investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	37
Other assets | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	37
Other assets | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total assets measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,357	1,317
Total assets measured at fair value | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	441	388
Total assets measured at fair value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	906	919
Total assets measured at fair value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Total limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Other assets	$ 904	$ 876