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Fair Value Measurements - Summary of Restricted Investment Securities Portfolio (Details) - USD ($) $ in Millions		Jun. 30, 2019	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	[1]	$ 288	$ 297
Total, fair value		329	342
Corporate bond mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		66	120
Unrealized gain		2	0
Unrealized loss		0	(5)
Total, fair value		68	115
Equity mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		71	148
Unrealized gain		29	59
Unrealized loss		0	0
Total, fair value		100	207
Demand Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Demand deposit		161	20
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		298	288
Unrealized gain		31	59
Unrealized loss		0	(5)
Total, fair value		$ 329	$ 342
[1] The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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