Employee Pension and Other Postretirement Benefits - Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in fair value of plan assets
Fair value of plan assets, beginning of year	$ 2,992	$ 2,897
Actual return on plan assets	221	13
Employer contributions	150	175
Plan participants' contributions	0	0
Benefits paid	(109)	(93)
Fair value of plan assets, end of year	$ 3,254	2,992
Pension Benefits \| Equity securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	63.00%
Pension Benefits \| Debt Securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	25.00%
Pension Benefits \| Real estate
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	12.00%
Other Benefits
Change in fair value of plan assets
Fair value of plan assets, beginning of year	$ 254	257
Actual return on plan assets	17	(2)
Employer contributions	9	16
Plan participants' contributions	8	7
Benefits paid	(28)	(24)
Fair value of plan assets, end of year	$ 260	$ 254
Other Benefits \| Equity securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	70.00%
Other Benefits \| Debt Securities
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation	30.00%

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)
-URI http://asc.fasb.org/extlink&oid=6504192

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)
-URI http://asc.fasb.org/extlink&oid=6504226

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=49170737

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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The amount of contributions made by plan participants. This item represents a periodic increase to the plan obligation and an increase to plan assets.

+ References

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Target allocation of investments of this type to total plan assets presented on a weighted average basis.

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