Retirement Plans (Fair Value Measurements Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Other Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, December 31	$ 232
Derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	402
Fair value of plan assets, December 31		$ 402
Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	10,312	10,923
Fair value of plan assets, December 31	11,368	10,312
Domestic Plan [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	(59)	(10)
Relating to assets still held at the reporting date	95	(65)
Relating to assets sold during the period	(23)	(24)
Purchases, sales and settlements	16	40
Transfers in and/or out of Level 3	0	0
Fair value of plan assets, December 31	29	(59)
Domestic Plan [Member] \| Other Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	245
Fair value of plan assets, December 31	(802)	245
Domestic Plan [Member] \| Other Fixed Income [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	11	10
Relating to assets still held at the reporting date	1	1
Relating to assets sold during the period	0	0
Purchases, sales and settlements	0	0
Transfers in and/or out of Level 3	0	0
Fair value of plan assets, December 31	12	11
Domestic Plan [Member] \| Derivatives [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	(71)
Fair value of plan assets, December 31	8	(71)
Domestic Plan [Member] \| Derivatives [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	(71)	(20)
Relating to assets still held at the reporting date	94	(66)
Relating to assets sold during the period	(23)	(24)
Purchases, sales and settlements	16	39
Transfers in and/or out of Level 3	0	0
Fair value of plan assets, December 31	16	(71)
Domestic Plan [Member] \| Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	41
Fair value of plan assets, December 31	1	41
Domestic Plan [Member] \| Mortgage Backed Securities [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, January 1	1	0
Relating to assets still held at the reporting date	0	0
Relating to assets sold during the period	0	0
Purchases, sales and settlements	0	1
Transfers in and/or out of Level 3	0	0
Fair value of plan assets, December 31	$ 1	$ 1

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(C)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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