Income Taxes - Differences Between Income Tax Provision (Benefit) and Income Taxes Computed using Applicable U.S. Statutory Federal Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Differences between the income tax provision (benefit) and income taxes computed using the applicable U.S. statutory federal tax rate
Income tax provision computed at federal statutory rate	$ 26,163	$ 2,217	$ 6,177
State income taxes, net of federal benefit	960	113	243
Foreign income taxed at different rates	(2,001)	(755)	(938)
Impact of equity investments	(2,499)	(1,666)	(1,069)
Change in deferred tax asset valuation allowance	(10,881)	77,531	(36)
Net increase (reduction) in uncertain tax positions	731	(1,543)	(1,207)
Nondeductible compensation	622	782	1,325
Tax credits	(1,412)	(1,786)	(1,741)
Travel and entertainment	266	274	314
Merger costs		503	228
Other	191	140	134
Income tax provision	$ 12,140	$ 75,810	$ 3,430