Income Taxes - Provision (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income tax benefit	$ (5,830)	$ (650)
Benefit related to stock compensation windfalls	3,700
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		300
Benefit related to remeasurement of net U.S. deferred tax liabilities		$ 700
U.S. statutory federal tax rate (as a percent)		21.00%