Acquisition - Unaudited Pro Forma (Details) - West Suburban acquired loans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Net interest income	$ 166,495	$ 165,283
Noninterest income	59,036	48,937
Net income attributable to Old Second Bancorp, Inc.	$ 67,779	$ 12,349