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Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($)
$ in Millions

12 Months Ended

Sep. 30, 2019

Sep. 30, 2018

Sep. 30, 2017

Schedule of Available-for-sale Securities [Line Items]

Original cost

$ 335

$ 335

Accumulated adjustments, net

(47)

(38)

Adjusted cost

^[1]

288

297

Investment income

10

^[2]

8

^[2]

$ 9

Net unrealized gain

^[3]

29

54

Realized gain

^[3]

32

6

Settlement funds

7

10

Disbursements

(32)

(33)

$ (27)

Fair value

334

342

Available-for-sale Securities [Member]

Schedule of Available-for-sale Securities [Line Items]

Adjusted cost

305

288

Fair value

334

342

Unrealized gain

31

59

Unrealized loss

(2)

(5)

Bond Mutual Fund [Member]

Schedule of Available-for-sale Securities [Line Items]

Adjusted cost

176

120

Fair value

183

115

Unrealized gain

7

0

Unrealized loss

0

(5)

Equity Mutual Fund [Member]

Schedule of Available-for-sale Securities [Line Items]

Adjusted cost

120

148

Fair value

142

207

Unrealized gain

24

59

Unrealized loss

(2)

0

Demand Deposits [Member]

Schedule of Available-for-sale Securities [Line Items]

Demand Deposit

$ 9

$ 20

[1]	The adjusted cost of the demand deposit includes accumulated investment income, realized gains, disbursements and settlements recorded in previous periods.
[2]	Investment income relates to the demand deposit and includes interest income as well as dividend income transferred from the equity and corporate bond mutual funds.
[3]	Presented under the original cost method.

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