Income Taxes - Income Tax (Benefit) Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current (benefit) provision:
Federal		$ (473,751)	$ (12,801)	$ 247,755
State and local		30,236	15,246	39,328
Foreign		94,213	81,989	69,972
Total current (benefit) provision		(349,302)	84,434	357,055
Deferred (benefit) provision:
Federal		(914,613)	61,819	(828,023)
State and local		(264,409)	(31,086)	33,887
Foreign		(365,949)	(2,196)	(1,388)
Total deferred (benefit) provision		(1,544,971)	28,537	(795,524)
(Benefit) provision for income taxes	$ (37,000)	$ (1,894,273)	$ 112,971	$ (438,469)