Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income (loss)	$ 62,612	$ (69,476)	$ 14,221
Weighted average common shares outstanding used in computing basic earnings (losses) per share (in shares)	69,575	68,507	67,411
Dilutive common stock options and restricted stock units (in shares)	910		1,138
Weighted average common shares outstanding used in computing diluted earnings (losses) per share (in shares)	70,485	68,507	68,549
Basic net income (loss) per share (in dollars per share)	$ 0.90	$ (1.01)	$ 0.21
Diluted net income (loss) per share (in dollars per share)	$ 0.89	$ (1.01)	$ 0.21