Fair Value Measurements - Summary of Financial Instruments Subject to Recurring Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions		Sep. 30, 2019		Sep. 30, 2018
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Assets
Cash and cash equivalents		$ 232		$ 294
Restricted investments		334	[1]	342	[2]
Investments of captive insurance company	[3]	5		3
Foreign currency derivatives		2		11
Total assets at fair value		573		650
Liabilities
Foreign currency derivatives		2		3
Estimate of Fair Value, Fair Value Disclosure [Member]
Assets
Cash and cash equivalents		232		294
Restricted investments		334	[1]	342	[2]
Investments of captive insurance company	[3]	5		3
Foreign currency derivatives		2		11
Total assets at fair value		573		650
Liabilities
Foreign currency derivatives		2		3
Fair Value, Inputs, Level 1 [Member]
Assets
Cash and cash equivalents		232		294
Restricted investments		334	[1]	342	[2]
Investments of captive insurance company	[3]	5		3
Foreign currency derivatives		0		0
Total assets at fair value		571		639
Liabilities
Foreign currency derivatives		0		0
Fair Value, Inputs, Level 2 [Member]
Assets
Cash and cash equivalents		0		0
Restricted investments		0	[1]	0	[2]
Investments of captive insurance company	[3]	0		0
Foreign currency derivatives		2		11
Total assets at fair value		2		11
Liabilities
Foreign currency derivatives		2		3
Fair Value, Inputs, Level 3 [Member]
Assets
Cash and cash equivalents		0		0
Restricted investments		0	[1]	0	[2]
Investments of captive insurance company	[3]	0		0
Foreign currency derivatives		0		0
Total assets at fair value		0		0
Liabilities
Foreign currency derivatives		$ 0		$ 0

[1] Included in restricted investments and $24 million within other current assets in the Consolidated Balance Sheets.

[2] Included in restricted investments and $30 million within other current assets in the Consolidated Balance Sheets.

[3] Included in other noncurrent assets in the Consolidated Balance Sheets.