Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Net (Loss) Income Per Share.
Net (loss) income	$ (213,999)	$ 16,479	$ 23,046	$ 19,730	$ 24,647	$ 28,772	$ 24,312	$ 24,370	$ (154,744)	$ 102,102	$ 104,812
Basic weighted average shares outstanding									96,955,043	95,950,994	93,285,490
Dilutive effect of stock options and restricted stock awards/units (in shares)										1,654,789	2,558,259
Dilutive weighted average shares outstanding									96,955,043	97,605,783	95,843,749
Basic net income per share (in dollars per share)	$ (2.21)	$ 0.17	$ 0.24	$ 0.20	$ 0.25	$ 0.30	$ 0.25	$ 0.26	$ (1.60)	$ 1.06	$ 1.12
Diluted net income per share (in dollars per share)	$ (2.21)	$ 0.17	$ 0.24	$ 0.20	$ 0.25	$ 0.29	$ 0.25	$ 0.25	$ (1.60)	$ 1.05	$ 1.09
Common stock equivalents not included in diluted calculation due to anti-dilutive effect (in shares)									2,292,770	510,800	0