Stockholders' Equity - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 717,832	$ 607,644	$ 553,690
Ending Balance	1,138,954	717,832	607,644
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	13,587	15,213	15,166
Other comprehensive (loss) income before reclassifications	(9,971)	(1,637)	283
Amounts reclassified from accumulated other comprehensive income	(105)	11	(236)
Ending Balance	3,511	13,587	15,213
Currency Translation Adjustments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	13,517	15,168	15,389
Other comprehensive (loss) income before reclassifications	(9,333)	(1,651)	(221)
Ending Balance	4,184	13,517	15,168
Unrealized Gains (Losses) on Available-for-Sale Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(112)	(1)	(3)
Other comprehensive (loss) income before reclassifications	244	(110)	(10)
Amounts reclassified from accumulated other comprehensive income	(140)	(1)	12
Ending Balance	(8)	(112)	(1)
Pension Liability Adjustments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	182	46	(220)
Other comprehensive (loss) income before reclassifications	(882)	124	514
Amounts reclassified from accumulated other comprehensive income	35	12	(248)
Ending Balance	$ (665)	$ 182	$ 46

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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