Postretirement Benefit Plans Postretirement Benefit Plans (Fair Value of Plan Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 562	$ 559
Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	6	4
Index funds
Defined Benefit Plan Disclosure
Fair value of plan assets	36	34
Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	18	11
Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	11	10
Hedge funds
Defined Benefit Plan Disclosure
Fair value of plan assets	178	181
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure
Fair value of plan assets	562	559
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	231	255
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	45	22
Estimate of Fair Value Measurement | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	24	25
Estimate of Fair Value Measurement | Global stock funds/securities
Defined Benefit Plan Disclosure
Fair value of plan assets	93	83
Estimate of Fair Value Measurement | Global stock funds/securities | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	87	79
Estimate of Fair Value Measurement | Global stock funds/securities | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Global stock funds/securities | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Index funds
Defined Benefit Plan Disclosure
Fair value of plan assets	40	40
Estimate of Fair Value Measurement | Index funds | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	4	6
Estimate of Fair Value Measurement | Index funds | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Index funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Emerging market funds
Defined Benefit Plan Disclosure
Fair value of plan assets	4	3
Estimate of Fair Value Measurement | Emerging market funds | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	4	3
Estimate of Fair Value Measurement | Emerging market funds | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Emerging market funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	40	49
Estimate of Fair Value Measurement | Corporate bonds | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	22	34
Estimate of Fair Value Measurement | Corporate bonds | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	4
Estimate of Fair Value Measurement | Corporate bonds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	99	127
Estimate of Fair Value Measurement | Government bonds | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	88	99
Estimate of Fair Value Measurement | Government bonds | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	18
Estimate of Fair Value Measurement | Government bonds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Hedging Instruments
Defined Benefit Plan Disclosure
Fair value of plan assets	45	0
Estimate of Fair Value Measurement | Hedging Instruments | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	45	0
Estimate of Fair Value Measurement | Hedge funds
Defined Benefit Plan Disclosure
Fair value of plan assets	178	190
Estimate of Fair Value Measurement | Hedge funds | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	9
Estimate of Fair Value Measurement | Hedge funds | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Hedge funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Insurance contracts and other
Defined Benefit Plan Disclosure
Fair value of plan assets	50	50
Estimate of Fair Value Measurement | Insurance contracts and other | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	26	25
Estimate of Fair Value Measurement | Insurance contracts and other | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Estimate of Fair Value Measurement | Insurance contracts and other | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 24	$ 25