Net Income Per Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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, 2019	Sep. 30, 2018	Sep. 30, 2017
Basic net income per share:
Net income	$ 9,191	$ 7,036	$ 4,145	$ 26,011	$ 27,420	$ 20,288	$ 12,555	$ 11,693	$ 46,383	$ 71,956	$ 17,710
Weighted average common shares outstanding — basic									19,584	19,255	19,066
Net income per share common share — basic	$ 0.47	$ 0.36	$ 0.21	$ 1.34	$ 1.41	$ 1.05	$ 0.65	$ 0.61	$ 2.37	$ 3.74	$ 0.93
Diluted net income per share:
Net income	$ 9,191	$ 7,036	$ 4,145	$ 26,011	$ 27,420	$ 20,288	$ 12,555	$ 11,693	$ 46,383	$ 71,956	$ 17,710
Weighted average common shares outstanding — basic									19,584	19,255	19,066
Dilutive effect of common stock equivalents									1,384	1,395	341
Weighted average common shares outstanding — diluted									20,968	20,650	19,407
Net income per share common share — diluted	$ 0.44	$ 0.33	$ 0.20	$ 1.25	$ 1.30	$ 0.97	$ 0.61	$ 0.59	$ 2.21	$ 3.48	$ 0.91
Anti-dilutive common stock equivalents excluded from above									843	295	2,161