Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018
Income Tax [Line Items]
Effective tax rate		27.70%	24.40%
Net income tax benefit related to exercise of stock options		$ (64,000)	$ (64,000)
Net income tax benefit related to vesting of restricted shares		$ (362,000)	$ (362,000)
Corporate federal income tax rate		21.00%		35.00%
Net income tax benefit	$ (724,000)
Deferred tax assets, net		$ 33,400,000			$ 35,600,000
Oak Park River Forest Bankshares, Inc.
Income Tax [Line Items]
Decrease in deferred tax assets, net		$ (4,900,000)