Fair Value Measurements (Postretirement Health Care Plan) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Level 1 | Invested cash
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	$ 0	$ 0
Level 2 | Invested cash
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	6
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	136	143
Postretirement Health Coverage | Common stock and equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	4	4
Postretirement Health Coverage | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	68	71
Postretirement Health Coverage | Government, agency, and government-sponsored enterprise obligations
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	5	5
Postretirement Health Coverage | Other government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	2	3
Postretirement Health Coverage | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	35	40
Postretirement Health Coverage | Commingled bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	14
Postretirement Health Coverage | Commingled short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	8	14
Postretirement Health Coverage | Invested cash
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	6
Postretirement Health Coverage | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	70	4
Postretirement Health Coverage | Level 1 | Common stock and equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	4	4
Postretirement Health Coverage | Level 1 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	47	0
Postretirement Health Coverage | Level 1 | Government, agency, and government-sponsored enterprise obligations
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Postretirement Health Coverage | Level 1 | Other government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Postretirement Health Coverage | Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Postretirement Health Coverage | Level 1 | Commingled bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	11
Postretirement Health Coverage | Level 1 | Commingled short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	8	0
Postretirement Health Coverage | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	66	139
Postretirement Health Coverage | Level 2 | Common stock and equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	0	0
Postretirement Health Coverage | Level 2 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	21	71
Postretirement Health Coverage | Level 2 | Government, agency, and government-sponsored enterprise obligations
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	5	5
Postretirement Health Coverage | Level 2 | Other government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	2	3
Postretirement Health Coverage | Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	35	40
Postretirement Health Coverage | Level 2 | Commingled bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	3
Postretirement Health Coverage | Level 2 | Commingled short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value plan assets	$ 0	$ 14