Derivatives and Hedging Activities - Narrative (Details)	3 Months Ended
	Dec. 31, 2017 USD ($) swap	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) swap
Derivative [Line Items]
Derivative gains (losses) recorded in the Statements of Income	$ 0	$ 0
Accrued interest on derivatives, at fair value	7,000		$ 9,000
Currency forward contract - non-designated hedge
Derivative [Line Items]
Notional amount of derivative	1,600,000		1,600,000
Interest rate swap - cash flow hedge
Derivative [Line Items]
Notional amount of derivative	$ 20,000,000		$ 20,000,000
Number of interest rate derivatives held | swap	2		2