Revenue Recognition - Contract Assets and Liabilities (Details) - USD ($) $ in Millions	2 Months Ended	10 Months Ended	12 Months Ended
	Dec. 15, 2017	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2017	Oct. 01, 2018
Revenue from Contract with Customer [Abstract]
Contract with Customer, Liability, Revenue Recognized			$ 537
Accounts receivable, net
Accounts receivable, net		$ 377	314		$ 376
Increase (decrease) in accounts receivable, net			(62)
Contract assets:
Contract with Customer, Asset, Net, Current		0	187		78
Increase (decrease) in contract assets, current			109
Contract assets, non-current			16		3
Increase (decrease) in contract assets, non-current			13
Total contract assets			203		81
Increase (decrease) in total contract assets	$ 0	0	122	$ 0
Cost of obtaining a contract:
Cost of obtaining a contract, current			89		64
Increase (decrease) in cost of obtaining a contract, current			25
Cost of obtaining a contract, non-current			45		36
Increase (decrease) in cost of obtaining a contract, non-current			9
Total cost of obtaining a contract			134		100
Increase (decrease) in total cost of obtaining a contract			34
Cost to fulfill a contract:
Cost incurred to fulfill a contract, current			25		45
Increase (decrease) in cost incurred to fulfill a contract, current			(20)
Contract liabilities:
Contract with Customer, Liability, Current		$ 484	472		467
Increase (decrease) in contract liabilities, current			5
Contract liabilities, non-current			78		52
Increase (decrease) in contract liabilities, non-current			26
Total contract liabilities			550		$ 519
Increase (decrease) in total contract liabilities			31
Capitalized Contract Cost to Obtain a Contract, Amortization			103
Capitalized Contract Cost to Fulfill, Amortization			$ 50