Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 33
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in current fiscal year	76
United States
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 34	$ 34
United States \| Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percentage	64.00%
Actual asset allocation percentage of equity	63.00%
United States \| Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percentage	36.00%
Actual asset allocation percentage of equity	37.00%
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 22	$ 20
United Kingdom \| Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percentage	34.00%
Actual asset allocation percentage of equity	37.00%
United Kingdom \| Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percentage	66.00%
Actual asset allocation percentage of equity	63.00%
United Kingdom \| Geographic Concentration Risk \| Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Concentration risk percentage	81.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

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Amount of cost for defined contribution plan.

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