Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Original cost		$ 335		$ 335
Accumulated adjustments, net		(38)		(24)
Adjusted cost	[1]	297		311
Investment income		8	[2]	9	[2]	$ 8
Net unrealized gain		54		35
Realized gain		6		2		0
Settlement funds		10		2
Disbursements		(33)		(27)		$ (33)
Fair value		342		332
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		288		297
Fair value		342		332
Unrealized gain		59		35
Unrealized loss		(5)		0
Corporate bond mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		120		120
Corporate bond mutual fund, unrealized gain		0		1
Corporate bond mutual fund, unrealized loss		(5)		0
Corporate bond mutual fund, Fair value		115		121
Equity mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		148		168
Equity mutual fund, unrealized gain		59		34
Equity mutual fund, unrealized loss		0		0
Equity mutual fund, Fair value		207		202
Demand Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Demand Deposit		$ 20		$ 9

[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.