Selected Consolidated Quarterly Financial Data (Unaudited) - Schedule of Selected Consolidated Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	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, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Line Items]
Total operating revenues	$ 187,830	$ 180,224	$ 177,147	$ 178,156	$ 177,532	$ 171,739	$ 167,640	$ 164,684	$ 157,293	$ 166,952	$ 159,096	$ 160,235	$ 723,357	$ 681,595	$ 643,576
Operating income	30,453	17,077	34,377	39,230	41,784	(508)	16,349	15,023	22,691	36,360	20,405	20,838	121,137	72,648	100,294
Net income	$ 12,243	$ 3,007	$ 13,249	$ 19,081	$ 19,394	$ (11,135)	$ 2,372	$ 22,624	$ 5,482	$ 15,432	$ 5,304	$ 6,610	$ 47,580	$ 33,255	$ 32,828
Net income per common share
Basic	$ 0.35	$ 0.09	$ 0.38	$ 0.55	$ 0.66	$ (0.48)	$ 0.10	$ 0.97	$ 0.24	$ 0.67	$ 0.23	$ 0.28	$ 1.37	$ 1.34	$ 1.41
Diluted	$ 0.35	$ 0.09	$ 0.38	$ 0.54	$ 0.65	$ (0.48)	$ 0.10	$ 0.96	$ 0.24	$ 0.66	$ 0.23	$ 0.28	$ 1.36	$ 1.32	$ 1.40
Contract Revenue [Member]
Quarterly Financial Information Disclosure [Line Items]
Total operating revenues	$ 172,248	$ 170,366	$ 169,771	$ 170,449	$ 168,444	$ 159,916	$ 156,515	$ 154,389	$ 151,575	$ 157,411	$ 154,210	$ 155,502	$ 682,834	$ 639,264	$ 618,698