Material Transactions Affecting Stockholders' Equity (Details) - $ / shares				3 Months Ended
	Oct. 12, 2018	May 17, 2017	Jan. 06, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Class of Stock [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01				$ 0.01
Common stock, increase shares authorized (in shares)		100,000,000
Common stock and preferred stock, shares authorized (in shares)		251,000,000
Preferred stock, shares authorized (in shares)		1,000,000		1,000,000				1,000,000
Common stock, shares authorized (in shares)		250,000,000		250,000,000				250,000,000
Common dividends declared per common share (in dollars per share)				$ 0.12	$ 0.11	$ 0.11	$ 0.11	$ 0.10	$ 0.10	$ 0.10	$ 0.09	$ 0.09	$ 0.09	$ 0.09	$ 0.09
Northern States Financial Corporation
Class of Stock [Line Items]
Common stock, shares issued (in shares)	3,310,912
Shares issued (in dollars per share)	$ 25.16
Standard Bancshares, Inc.
Class of Stock [Line Items]
Common stock, shares issued (in shares)			21,057,085
Shares issued (in dollars per share)			$ 25.34