Income Taxes - Provision (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income tax expense (benefit)	$ 1,030	$ 54,531	$ 6,860	$ 55,181
Benefit related to stock compensation windfalls	$ 400		4,100
Benefit related to remeasurement of net U.S. deferred tax assets			1,400
Benefit related to transition tax reduction			$ 1,100
Benefit related to reduction of reserves		$ 58,000
Benefit related to remeasurement of net U.S. deferred tax liabilities				$ 700
U.S. statutory federal tax rate (as a percent)				21.00%