Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liabilities	$ 359
Operating lease right-of-use assets	289
Deferred rent liability		$ 69
Decrease to accumulated deficit	$ (1,088)		$ (1,177)
ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liabilities		366
Operating lease right-of-use assets		297
Decrease to accumulated deficit		$ 7