Employee Postretirement Benefits (Principal Plans and All Other Pension Plans) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Principal Plans
Defined Benefit Plan, Benefit Obligation	$ 3,567	$ 3,427
Defined Benefit Plan, Accumulated Benefit Obligation	3,513	3,378
Fair value of plan assets	3,312	3,011
All Other Pension Plans
Defined Benefit Plan, Benefit Obligation	729	699
Defined Benefit Plan, Accumulated Benefit Obligation	658	622
Fair value of plan assets	585	523
Defined Benefit Pension Plans
Defined Benefit Plan, Benefit Obligation	4,296	4,126	$ 3,959
Defined Benefit Plan, Accumulated Benefit Obligation	4,171	4,000
Fair value of plan assets	$ 3,897	$ 3,534	$ 3,508

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Projected Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6522206

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Accumulated Postretirement Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6503904

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