Acquisitions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Aug. 27, 2019	May 09, 2019	Oct. 12, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Jan. 16, 2019	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill				$ 796,756	$ 728,809	$ 697,608			$ 340,879
Acquisition and integration related expenses				$ 21,860	$ 9,613	$ 20,123
Park Bank
Business Acquisition [Line Items]
Announced assets to be acquired							$ 1,000,000
Announced deposits to be acquired							875,000
Announced loans to be acquired							$ 700,000
Fixed exchange ratio (in shares)	29.9675
Cash paid per share in business acquisition	$ 623.02
Announced consideration to be transferred	$ 195,000
Bridgeview Bancorp, Inc.
Business Acquisition [Line Items]
Fixed exchange ratio (in shares)		0.2767
Cash paid per share in business acquisition		$ 1.66
Total assets		$ 1,165,133
Total deposits		986,754
Total loans		710,647
Consideration transferred		$ 135,352
Common stock, shares issued (in shares)		4,728,541
Cash paid		$ 37,140
Goodwill		$ 59,142
Northern Oak Wealth Management, Inc.
Business Acquisition [Line Items]
Assets under management acquired								$ 800,000
Northern States Financial Corporation
Business Acquisition [Line Items]
Fixed exchange ratio (in shares)			0.000363
Total assets			$ 579,333
Total deposits			463,160
Total loans			284,940
Consideration transferred			$ 83,303
Common stock, shares issued (in shares)			3,310,912
Cash paid			$ 0
Goodwill			$ 30,016