Employee Pension and Other Postretirement Benefits - Investments Measured at Net Asset Value (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 680,000,000	$ 1,420,000,000
Unfunded Commitment	$ 2,000,000	3,000,000
Pension Benefits \| Equity Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		$ 789,000,000
Redemption Notice Period	15 days	15 days
Pension Benefits \| Real estate funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 480,000,000	$ 415,000,000
Unfunded Commitment	$ 2,000,000
Redemption Notice Period	90 days
Pension Benefits \| Real estate funds \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period	45 days	45 days
Pension Benefits \| Real estate funds \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Redemption Notice Period		90 days
Pension Benefits \| Real estate funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 5,000,000	$ 7,000,000
Unfunded Commitment	0	3,000,000
Pension Benefits \| International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value	$ 195,000,000	159,000,000
Pension Benefits \| Money market funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		50,000,000
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		134,000,000
Other Benefits \| Equity Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		$ 87,000,000
Redemption Notice Period	7 days	7 days
Other Benefits \| Pooled separate account - variable life insurance policies
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		$ 46,000,000
Other Benefits \| Money market funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		1,000,000
Underfunded \| Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		0
Underfunded \| Other Benefits \| Equity Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		0
Underfunded \| Other Benefits \| Pooled separate account - variable life insurance policies
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		0
Underfunded \| Other Benefits \| Money market funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value		$ 0

Defined Benefit Plan, Plan Asset, Investment Unfunded Commitment

No definition available.

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- Definition

Amount of employer security in which defined benefit plan asset is invested. Includes, but is not limited to, security issued or managed by related party of employer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (n)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

+ Details

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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.

+ Details

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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=117815213&loc=SL6742756-110258

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