Summary of Quarterly Results of Operations (Unaudited) - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Summary Of quarterly results of operations
Increase (decrease) in operating income	$ 27,673	$ 10,290	$ (1,679)	$ (11,902)	$ 21,207	$ (6,872)	$ (6,084)	$ (5,623)	$ 24,382	$ 2,628	$ (11,649)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									12,036	(11,209)	1,735
Net income (loss) after taxes	$ 17,816	$ 6,291	$ (2,011)	$ (9,786)	$ 13,155	$ (21,957)	$ 461	$ (2,868)	$ 12,310	$ (11,209)	$ 1,735
Increase (decrease) in net income per common share (in dollars per share)	$ 0.65	$ 0.23	$ (0.07)	$ (0.36)	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ 0.45	$ (0.41)	$ 0.06
Change in estimated total costs | Adjustment
Summary Of quarterly results of operations
Increase (decrease) in operating income	$ (4,200)				$ (1,600)				$ (7,000)	$ 5,700	$ (900)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ (3,100)				$ (1,100)				$ (5,100)	$ 3,200	$ (500)
Increase (decrease) in net income per common share (in dollars per share)	$ (0.12)				$ (0.04)				$ (0.19)	$ 0.12	$ (0.02)