Employee Benefit Plans - Schedule of Investments Measured at Fair Value Using NAV Per Share (Details) $ in Millions	12 Months Ended
Sep. 30, 2019 USD ($)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	$ 489
Unfunded commitments	$ 2
Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period in which private equity investments are estimated to be liquidated	1 year
Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period in which private equity investments are estimated to be liquidated	5 years
Long/short hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	$ 3
Unfunded commitments	0
Relative value hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	4
Unfunded commitments	0
Multi-strategy hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	0
Unfunded commitments	0
Event driven hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	1
Unfunded commitments	0
Common collective trusts, daily redemption
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	453
Unfunded commitments	$ 0
Redemption notice period	3 days
Common collective trusts, monthly redemption
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	$ 13
Unfunded commitments	$ 0
Redemption notice period	5 days
Common collective trusts
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	$ 8
Unfunded commitments	0
Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value at NAV	7
Unfunded commitments	$ 2

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=117332851&loc=SL7495116-110257

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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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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.

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Remaining period of expected liquidation by investee of underlying asset of investment measured at net asset value per share or unit. In 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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
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-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

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