Employee Benefit Plans - Investments Measured at NAV (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Hedge funds and hedge fund-related strategies \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	120 days	120 days
Hedge funds and hedge fund-related strategies \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	2 days	4 days
Commingled funds, private equity and private equity-related instruments \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	30 days	30 days
Commingled funds, private equity and private equity-related instruments \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	10 days	15 days
Fixed income and fixed income-related instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period		5 days
Fixed income and fixed income-related instruments \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	15 days
Fixed income and fixed income-related instruments \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	3 days
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	30 days	2 days
Fair Value, Measurements, Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 7,378	$ 5,724
Unfunded Commitments	94	1,054
Fair Value, Measurements, Recurring \| Hedge funds and hedge fund-related strategies
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	4,768	3,308
Unfunded Commitments	0	0
Fair Value, Measurements, Recurring \| Commingled funds, private equity and private equity-related instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	1,375	1,214
Unfunded Commitments	0	525
Fair Value, Measurements, Recurring \| Fixed income and fixed income-related instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	311	270
Unfunded Commitments	0	0
Fair Value, Measurements, Recurring \| Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	924	698
Unfunded Commitments	94	529
Fair Value, Measurements, Recurring \| Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	0	234
Unfunded Commitments	$ 0	$ 0

Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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