Supplemental Guarantor Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Condensed Cash Flow Statements, Captions [Line Items]
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 290	$ 51
INVESTING ACTIVITIES
Capital expenditures	(33)	(44)
Cash paid for acquisition of Transportation Power, Inc., net of cash acquired	(13)	(3)
Other investing activities	9	0
CASH USED FOR INVESTING ACTIVITIES	(37)	(47)
FINANCING ACTIVITIES
Securitization	96	48
Borrowings against revolving line of credit	304	0
Redemption of notes	0	(19)
Term loan payments	(4)	0
Repurchase of common stock	(241)	(50)
Intercompany advances	0	0
Other financing activities	(1)	(1)
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	154	(22)
EFFECT OF CHANGES IN FOREIGN CURRENCY EXCHANGE RATES ON CASH AND CASH EQUIVALENTS	(7)	1
CHANGE IN CASH AND CASH EQUIVALENTS	400	(17)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	108	115
CASH AND CASH EQUIVALENTS AT END OF PERIOD	508	98
Eliminations
Condensed Cash Flow Statements, Captions [Line Items]
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	0	0
INVESTING ACTIVITIES
Capital expenditures	0	0
Cash paid for acquisition of Transportation Power, Inc., net of cash acquired	0	0
CASH USED FOR INVESTING ACTIVITIES	0	0
FINANCING ACTIVITIES
Securitization	0	0
Borrowings against revolving line of credit	0
Redemption of notes		0
Term loan payments	0
Repurchase of common stock	0	0
Intercompany advances	0	0
Other financing activities	0	0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	0	0
EFFECT OF CHANGES IN FOREIGN CURRENCY EXCHANGE RATES ON CASH AND CASH EQUIVALENTS	0	0
CHANGE IN CASH AND CASH EQUIVALENTS	0	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	0
Parent | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	370	32
INVESTING ACTIVITIES
Capital expenditures	(1)	(2)
Cash paid for acquisition of Transportation Power, Inc., net of cash acquired	(13)	(3)
Other investing activities	9
CASH USED FOR INVESTING ACTIVITIES	(5)	(5)
FINANCING ACTIVITIES
Securitization	0	0
Borrowings against revolving line of credit	304
Redemption of notes		(19)
Term loan payments	(4)
Repurchase of common stock	(241)	(50)
Intercompany advances	(76)	29
Other financing activities	(1)	0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	(18)	(40)
EFFECT OF CHANGES IN FOREIGN CURRENCY EXCHANGE RATES ON CASH AND CASH EQUIVALENTS	0	0
CHANGE IN CASH AND CASH EQUIVALENTS	347	(13)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	4	24
CASH AND CASH EQUIVALENTS AT END OF PERIOD	351	11
Guarantors | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	17	23
INVESTING ACTIVITIES
Capital expenditures	(17)	(23)
Cash paid for acquisition of Transportation Power, Inc., net of cash acquired	0	0
Other investing activities	0
CASH USED FOR INVESTING ACTIVITIES	(17)	(23)
FINANCING ACTIVITIES
Securitization	0	0
Borrowings against revolving line of credit	0
Redemption of notes		0
Term loan payments	0
Repurchase of common stock	0	0
Intercompany advances	0	0
Other financing activities	0	(1)
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	0	(1)
EFFECT OF CHANGES IN FOREIGN CURRENCY EXCHANGE RATES ON CASH AND CASH EQUIVALENTS	0	0
CHANGE IN CASH AND CASH EQUIVALENTS	0	(1)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	4	6
CASH AND CASH EQUIVALENTS AT END OF PERIOD	4	5
Non- Guarantors | Reportable Legal Entities
Condensed Cash Flow Statements, Captions [Line Items]
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(97)	(4)
INVESTING ACTIVITIES
Capital expenditures	(15)	(19)
Cash paid for acquisition of Transportation Power, Inc., net of cash acquired	0	0
Other investing activities	0
CASH USED FOR INVESTING ACTIVITIES	(15)	(19)
FINANCING ACTIVITIES
Securitization	96	48
Borrowings against revolving line of credit	0
Redemption of notes		0
Term loan payments	0
Repurchase of common stock	0	0
Intercompany advances	76	(29)
Other financing activities	0	0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	172	19
EFFECT OF CHANGES IN FOREIGN CURRENCY EXCHANGE RATES ON CASH AND CASH EQUIVALENTS	(7)	1
CHANGE IN CASH AND CASH EQUIVALENTS	53	(3)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	100	85
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ 153	$ 82