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Selected Consolidated Quarterly Financial Data (unaudited) (Detail) - 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
Quarterly Financial Information Disclosure [Line Items]
Total operating revenues	$ 177,532	$ 171,739	$ 167,640	$ 164,684	$ 157,293	$ 166,952	$ 159,096	$ 160,235	$ 681,595	$ 643,576	$ 587,836
Operating income	41,784	(508)	16,349	15,023	22,691	36,360	20,405	20,838	72,648	100,294	56,758
Net income	$ 19,394	$ (11,135)	$ 2,372	$ 22,624	$ 5,482	$ 15,432	$ 5,304	$ 6,610	$ 33,255	$ 32,828	$ 14,920
Net income per common share
Basic	$ 1.04	$ (0.89)	$ 0.20	$ 2.00	$ 0.49	$ 1.40	$ 0.48	$ 0.62	$ 2.46	$ 3.01	$ 1.56
Diluted	$ 0.65	$ (0.89)	$ 0.10	$ 0.96	$ 0.23	$ 0.66	$ 0.23	$ 0.28	$ 1.32	$ 1.40	$ 0.62
Contract Revenue [Member]
Quarterly Financial Information Disclosure [Line Items]
Total operating revenues	$ 168,444	$ 159,916	$ 156,515	$ 154,389	$ 151,575	$ 157,411	$ 154,210	$ 155,502	$ 639,264	$ 618,698	$ 569,373

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