Pension Plans and Other Postretirement Benefits - Change in Plan Assets (Details) - Pension Plans - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets
Fair value of plan assets at the beginning of the year	$ 624	$ 613
Actual return on plan assets	32	26
Employer contributions	46	25	$ 22
Benefit payments	(35)	(37)
Settlements	(667)	(3)
Fair value of plan assets at the end of the year		624	$ 613
Funded status	$ (131)	$ (220)

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
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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

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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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No definition available.

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- Definition

Amount of contributions made by the employer to defined benefit plans.

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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.

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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Amount of plan assets that relate to an irrevocable action that relieves the employer, or plan, of primary responsibility for a benefit obligation and eliminates risks related to the obligation and the assets used to effect settlement. Includes, but is not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified pension benefits and purchasing annuity contracts to cover vested benefits.

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