Supplemental Disclosure for Earnings Per Share (Earnings Per Share Information) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Earnings:
Net income attributable to First Savings Financial Group, Inc.	$ 2,931	$ 3,406
Shares:
Weighted average shares outstanding	2,284,665	2,228,256
Net income per share, basic	$ 1.28	$ 1.53
Earnings:
Net income attributable to First Savings Financial Group, Inc.	$ 2,931	$ 3,406
Shares:
Weighted average shares outstanding	2,284,665	2,228,256
Add: Dilutive effect of outstanding options	81,210	124,466
Add: Dilutive effect of nonvested restricted stock	5,605	6,213
Weighted average shares outstanding, as adjusted	2,371,480	2,358,935
Net income per share, diluted	$ 1.24	$ 1.44