Earnings per Share - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 67,020	$ 14,005	$ 83,506	$ 27,876
Weighted average common shares outstanding used in computing basic earnings (losses) per share (in shares)	70,220	69,600	70,340	69,388
Dilutive common stock options and restricted stock units (in shares)	393	549	568	685
Weighted average common shares outstanding used in computing diluted earnings (losses) per share (in shares)	70,613	70,149	70,908	70,073
Basic net income per share (in dollars per share)	$ 0.95	$ 0.20	$ 1.19	$ 0.40
Diluted net income per share (in dollars per share)	$ 0.95	$ 0.20	$ 1.18	$ 0.40