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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, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
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, 2020
Sep. 30, 2019
Sep. 30, 2018
Quarterly Financial Information Disclosure [Line Items]                              
Total operating revenues $ 108,039 $ 73,099 $ 179,896 $ 184,036 $ 187,830 $ 180,224 $ 177,147 $ 178,156 $ 177,532 $ 171,739 $ 167,640 $ 164,684 $ 545,070 $ 723,357 $ 681,595
Operating income 23,864 15,224 13,892 27,187 30,453 17,077 34,377 39,230 41,784 (508) 16,349 15,023 80,167 121,137 72,648
Net income $ 11,375 $ 3,419 $ 1,885 $ 10,785 $ 12,243 $ 3,007 $ 13,249 $ 19,081 $ 19,394 $ (11,135) $ 2,372 $ 22,624 $ 27,464 $ 47,580 $ 33,255
Net income per share                              
Basic $ 0.32 $ 0.10 $ 0.05 $ 0.31 $ 0.35 $ 0.09 $ 0.38 $ 0.55 $ 0.66 $ (0.48) $ 0.10 $ 0.97 $ 0.78 $ 1.37 $ 1.34
Diluted $ 0.32 $ 0.10 $ 0.05 $ 0.31 $ 0.35 $ 0.09 $ 0.38 $ 0.54 $ 0.65 $ (0.48) $ 0.10 $ 0.96 $ 0.78 $ 1.36 $ 1.32
Contract Revenue [Member]                              
Quarterly Financial Information Disclosure [Line Items]                              
Total operating revenues $ 97,361 $ 71,648 $ 165,781 $ 171,800 $ 172,248 $ 170,366 $ 169,771 $ 170,449 $ 168,444 $ 159,916 $ 156,515 $ 154,389 $ 506,590 $ 682,834 $ 639,264