Benefit Plans - Weighted average target allocation and actual percentage (Details) - Pension benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	100.00%	100.00%
Equity securities
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	58.00%	62.00%
Equity securities \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	43.00%
Equity securities \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	62.00%
Fixed income securities
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	34.00%	29.00%
Fixed income securities \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	34.00%
Fixed income securities \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	44.00%
Derivatives
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	1.00%
Securities pledged as collateral
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Assets for plan benefits	$ 202	$ 143
Limited partnership interests
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	6.00%	7.00%
Limited partnership interests \| Minimum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	0.00%
Limited partnership interests \| Maximum
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	13.00%
Short-term investments and other
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	0.00%
Actual percentage of plan assets	2.00%	2.00%
Private equity funds
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Actual percentage of plan assets	2.00%	1.00%
Mutual funds
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets
Target asset allocation	3.00%	3.00%

Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
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-URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1731-114919

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
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-Section S99
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-Subparagraph (SX 210.7-03(a)(10))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- References

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- Definition

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.

+ References

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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