Derivative Instruments and Hedging Activities - Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative asset	$ 26		$ 14
Derivative liability	1,122		$ 1,100
Foreign currency exchange derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset	17	$ 8
Foreign currency exchange derivatives | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset	1	1
Foreign currency exchange derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability	5	12
Foreign currency exchange derivatives | Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Derivative liability		3
Interest rate cap | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset		1
Cross-currency interest rate swaps | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset	10	12
Cross-currency interest rate swaps | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset	2	1
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815
Derivatives, Fair Value [Line Items]
Derivative asset	29	19
Derivative liability	1,126	1,109
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Long-term debt
Derivatives, Fair Value [Line Items]
Derivative liability, noncurrent	1,121	1,094
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, current	16	5
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, noncurrent	1	0
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, current	5	12
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, noncurrent	0	3
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Interest rate cap | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, noncurrent	0	1
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Cross-currency interest rate swaps | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, current	10	12
Derivatives and Hedging Activities Designated as Hedging Instruments under ASC 815 | Cross-currency interest rate swaps | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Hedging assets, noncurrent	2	1
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815
Derivatives, Fair Value [Line Items]
Derivative asset	1	4
Derivative liability	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Long-term debt
Derivatives, Fair Value [Line Items]
Derivative liability, noncurrent	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, current	1	3
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, noncurrent	0	1
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, current	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Foreign currency exchange derivatives | Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, noncurrent	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Interest rate cap | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Derivative asset, noncurrent	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Cross-currency interest rate swaps | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, current	0	0
Derivatives and Hedging Activities Not Designated as Hedging Instruments under ASC 815 | Cross-currency interest rate swaps | Other noncurrent assets
Derivatives, Fair Value [Line Items]
Hedging assets, noncurrent	$ 0	$ 0