Acquisitions - Summary of Fair Values of Assets and Liabilities Assumed as of Acquisition Dates (Details) - USD ($) $ in Thousands	May 15, 2019	Apr. 30, 2019	May 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Consideration paid
Goodwill				$ 145,638	$ 145,638	$ 128,177	$ 127,536	$ 127,536	$ 54,562
Oak Park River Forest Bankshares, Inc.
Assets
Cash and cash equivalents		$ 10,469
Securities available-for-sale		30,343
Restricted stock		414
Loans		261,151
Premises and equipment		3,488
Other real estate owned		2,201
Other intangible assets		6,220
Bank-owned life insurance		3,485
Deferred tax assets, net		4,887
Other assets		1,256
Total assets acquired		323,914
Liabilities
Deposits		290,171
Line of credit		5,655
Federal Home Loan Bank advances		5,300
Accrued expenses and other liabilities		4,766
Total liabilities assumed		305,892
Net assets acquired		18,022
Consideration paid
Common stock (2019 - 1,464,558 shares issued at $20.02 per share, 2018 - 6,682,850 shares issued at $21.62 per share)		29,320
Consideration paid in cash	$ 4,200	6,163
Total consideration paid		35,483
Goodwill		$ 17,461
First Evanston Bancorp, Inc.
Assets
Cash and cash equivalents			$ 47,378
Securities available-for-sale			128,063
Restricted stock			1,360
Loans			916,011
Premises and equipment			15,890
Other intangible assets			22,276
Deferred tax assets, net			2,302
Other assets			8,845
Total assets acquired			1,142,125
Liabilities
Deposits			1,022,268
Junior subordinated debentures			8,497
Accrued expenses and other liabilities			5,844
Total liabilities assumed			1,036,609
Net assets acquired			105,516
Consideration paid
Common stock (2019 - 1,464,558 shares issued at $20.02 per share, 2018 - 6,682,850 shares issued at $21.62 per share)			144,483
Outstanding stock options converted to Byline stock options			7,644
Consideration paid in cash			27,004
Total consideration paid			179,131
Goodwill			$ 73,615