Earnings per Share Reconciliation of Weighted Average Common Shares Outstanding for Purposes of Calculating Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net income (loss)	$ 14,005	$ (83,939)	$ 27,876	$ (88,587)
Weighted average common shares outstanding used in computing basic earnings (losses) per share	69,600	68,556	69,388	68,342
Dilutive restricted stock units	549	0	685	0
Weighted average common shares outstanding used in computing diluted earnings (losses) per share	70,149	68,556	70,073	68,342
Basic net income (loss) per share (USD per share)	$ 0.20	$ (1.22)	$ 0.40	$ (1.30)
Diluted net income (loss) per share (USD per share)	$ 0.20	$ (1.22)	$ 0.40	$ (1.30)