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Quarterly Financial Information - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information [Line Items]
Depreciation									$ 71	$ 61	$ 55
Quarterly Financial Information
Quarterly Financial Information [Line Items]
Depreciation	$ 18	$ 15	$ 14	$ 24	$ 18	$ 15	$ 14	$ 14
Common Class A
Quarterly Financial Information [Line Items]
Earnings per share, basic and diluted (in usd per share)									$ (0.82)	$ 0	$ 0
Common Class A | Quarterly Financial Information
Quarterly Financial Information [Line Items]
Earnings per share, basic and diluted (in usd per share)	$ 0.00	$ (1.03)	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Weighted average number of shares issued, basic and diluted (in shares)	87,280,769	20,307,692	0	0	0	0	0	0
Common Class B
Quarterly Financial Information [Line Items]
Earnings per share, basic and diluted (in usd per share)									$ (0.95)	$ 0.51	$ 0.61
Common Class B | Quarterly Financial Information
Quarterly Financial Information [Line Items]
Earnings per share, basic and diluted (in usd per share)	$ 0.00	$ (1.03)	$ (0.15)	$ 0.24	$ 0.18	$ 0.03	$ 0.13	$ 0.17
Weighted average number of shares issued, basic and diluted (in shares)	422,719,231	483,796,267	501,991,944	501,991,944	501,991,944	501,991,944	501,991,944	501,991,944

X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121326447&loc=d3e1337-109256

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