Conversion and Reorganization (Detail Textuals) $ / shares in Units, $ in Millions	Jul. 13, 2016 USD ($) $ / shares shares	Sep. 30, 2016 $ / shares	Dec. 31, 2015 $ / shares
Basis Of Accounting [Line Items]
Common stock - par value (in dollars per share) | $ / shares		$ 0.01	$ 0.10
FSB Bancorp, Inc.
Basis Of Accounting [Line Items]
Number of common stock issued | shares	1,034,649
Offering price (in dollars per share) | $ / shares	$ 10.00
Gross offering proceeds from stock offering | $	$ 10.3
Common stock - par value (in dollars per share) | $ / shares	$ 0.01
Conversion related expenses | $	$ 1.4
Net proceeds from stock offering | $	$ 8.9
Number of shares received | shares	1.0884
Conversion ratio on shares outstanding	1.08841
Ownership percentage	53.00%