Acquisitions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Dec. 06, 2018	Oct. 12, 2018	Jan. 06, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 16, 2019	Sep. 30, 2018	Feb. 28, 2017	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill				$ 728,809	$ 697,608	$ 340,879				$ 319,007
Acquisition and integration related expenses				9,613	$ 20,123	$ 14,352
Bridgeview Bancorp, Inc.
Business Acquisition [Line Items]
Announced assets to be acquired								$ 1,200,000
Announced deposits to be acquired	$ 1,100,000
Announced loans to be acquired	$ 800,000
Fixed exchange ratio (in shares)	0.2767
Cash paid per share in business acquisition	$ 1.79
Announced consideration to be transferred	$ 145,000
Northern States Financial Corporation
Business Acquisition [Line Items]
Fixed exchange ratio (in shares)		0.0363
Total assets		$ 578,669
Total deposits		463,160
Total loans		284,924
Consideration transferred		$ 83,303
Common stock, shares issued (in shares)		3,310,912
Goodwill		$ 29,343
Cash paid		$ 0
Premier Asset Management LLC
Business Acquisition [Line Items]
Assets under management acquired									$ 550,000
Measurement period adjustment to increase goodwill				$ 1,900
Standard Bancshares, Inc.
Business Acquisition [Line Items]
Fixed exchange ratio (in shares)			0.4350
Total assets			$ 2,639,060
Total deposits			2,023,874
Total loans			1,762,303
Consideration transferred			$ 580,715
Common stock, shares issued (in shares)			21,057,085
Goodwill			$ 345,334
Cash paid			$ 47,125
Subsequent Event | Northern Oak Wealth Management, Inc.
Business Acquisition [Line Items]
Assets under management acquired							$ 800,000