Fair Value Measurements - Summary of Restricted Investment Securities Portfolio (Details) - USD ($) $ in Millions		Dec. 31, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	[1]	$ 288	$ 297
Total, fair value		309	342
Corporate bond mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		120	120
Unrealized gain		0	0
Unrealized loss		(6)	(5)
Total, fair value		114	115
Equity mutual fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		148	148
Unrealized gain		30	59
Unrealized loss		0	0
Total, fair value		178	207
Demand Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Demand deposit		17	20
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		285	288
Unrealized gain		30	59
Unrealized loss		(6)	(5)
Total, fair value		$ 309	$ 342

[1] The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.