Share Repurchase Program (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 17, 2014
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program expiration date	Dec. 31, 2015
Stock purchased under repurchase program (in shares)			806,018
Stock purchased under repurchase program, value		$ 3,740,000	$ 25,000,000	$ 21,259,000
Stock purchased under repurchased program, average purchase price per share (in dollars per share)			$ 31.02
Maximum
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount					$ 25,000,000