Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Long-Term Debt Additional Details (Details)	12 Months Ended
Long-Term Debt Additional Details (Details)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2020 EUR (€)
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	$ 262,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	432,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,073,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	484,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	567,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	5,004,000,000
Interest Paid, Excluding Capitalized Interest, Operating Activities	247,000,000	$ 369,000,000	$ 401,000,000
Gain (Loss) on Extinguishment of Debt	0	(60,000,000)	$ 0
Johnson Controls International plc (JCI) and Tyco Fire & Security Finance S.C.A. (TFSCA) \| Zero Point Three Seven Five Percent Due Two Thousand Twenty Seven
Debt Instrument [Line Items]
Debt instrument, face amount \| €				€ 500,000,000
Long-term Debt	$ 583,000,000	0
Debt Instrument, Interest Rate, Stated Percentage	0.375%			0.375%
Johnson Controls International plc (JCI) and Tyco Fire & Security Finance S.C.A. (TFSCA) \| One Point Seven Five Percent Due Two Thousand Thirty
Debt Instrument [Line Items]
Debt instrument, face amount	$ 625,000,000
Long-term Debt	$ 623,000,000	0
Debt Instrument, Interest Rate, Stated Percentage	1.75%			1.75%
Johnson Controls International plc (JCI) and Tyco Fire & Security Finance S.C.A. (TFSCA) \| One Point Zero Percent Due Two Thousand Thirty Two
Debt Instrument [Line Items]
Debt instrument, face amount \| €				€ 500,000,000
Long-term Debt	$ 584,000,000	$ 0
Debt Instrument, Interest Rate, Stated Percentage	1.00%			1.00%