Employee Postretirement Benefits (Principal Plans and All Other Pension Plans) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Principal Plans
Defined Benefit Plan, Benefit Obligation	$ 3,427	$ 3,295
Defined Benefit Plan, Accumulated Benefit Obligation	3,378	3,253
Fair value of plan assets	3,011	3,019
All Other Pension Plans
Defined Benefit Plan, Benefit Obligation	699	664
Defined Benefit Plan, Accumulated Benefit Obligation	622	594
Fair value of plan assets	523	489
Defined Benefit Pension Plans
Defined Benefit Plan, Benefit Obligation	4,126	3,959	$ 6,860
Defined Benefit Plan, Accumulated Benefit Obligation	4,000	3,847
Fair value of plan assets	$ 3,534	$ 3,508	$ 5,914

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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