Stock-Based Compensation Plans - Schedule of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning balance	827,325	1,118,008	1,210,619
Number of Shares, Granted	229,039	241,582	164,401
Number of Shares, Exercised	(6,819)	(437,850)	(166,422)
Number of Shares, Forfeited and expired	(47,640)	(94,415)	(90,590)
Number of Shares Outstanding, Ending balance	1,001,905	827,325	1,118,008
Number of Shares, Exercisable	547,051
Weighted Average Exercise Price, Beginning Balance	$ 21.30	$ 20.10	$ 20.35
Weighted Average Exercise Price, Granted	24.08	20.63	21.24
Weighted Average Exercise Price, Exercised	13.89	15.68	14.54
Weighted Average Exercise Price, Forfeited and expired	30.65	31.52	35.35
Weighted Average Exercise Price, Ending balance	21.54	$ 21.30	$ 20.10
Weighted Average Exercise Price, Exercisable	$ 20.80