Material Transactions Affecting Stockholders' Equity (Details) - USD ($)				3 Months Ended												12 Months Ended
	May 09, 2019	Oct. 12, 2018	May 17, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Feb. 19, 2020	Mar. 19, 2019
Class of Stock [Line Items]
Stock repurchase program, authorized amount																		$ 180,000,000
Common stock repurchased (in shares)																1,700,000
Total cost of common stock repurchased																$ 33,928,000
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								1,000,000
Common stock, shares authorized (in shares)			250,000,000	250,000,000				250,000,000								250,000,000
Common stock, par value (in dollars per share)			$ 0.01	$ 0.01				$ 0.01								$ 0.01
Common dividends declared per common share (in dollars per share)				$ 0.14	$ 0.14	$ 0.14	$ 0.12	$ 0.12	$ 0.11	$ 0.11	$ 0.11	$ 0.10	$ 0.10	$ 0.10	$ 0.09
Subsequent Event
Class of Stock [Line Items]
Stock repurchase program, authorized amount																	$ 200,000,000
Stock repurchase program replaced																	$ 180,000,000
Bridgeview Bancorp, Inc.
Class of Stock [Line Items]
Common stock, shares issued (in shares)	4,728,541
Shares issued (in dollars per share)	$ 28.61
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