Employee Benefit Plans - Investments Measured at NAV (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 9,136	$ 7,378
Hedge funds and hedge fund-related strategies \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	180 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	2 days
Hedge funds and hedge fund-related strategies \| Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 5,264	$ 4,768
Commingled funds, private equity and private equity-related instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments	$ 490
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	2 days	10 days
Commingled funds, private equity and private equity-related instruments \| Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 1,591	$ 1,375
Fixed income and fixed income-related instruments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period		15 days
Unfunded commitments	$ 256
Fixed income and fixed income-related instruments \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	90 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	15 days	3 days
Fixed income and fixed income-related instruments \| Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 769	$ 311
Real assets
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments	227
Real assets \| Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 807	$ 924
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period		30 days
Other \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	90 days
Other \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	2 days
Other \| Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 705	$ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=99377789&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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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