Goodwill and Other Intangible Assets - Goodwill (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) segment	Jan. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 02, 2017 USD ($)
Goodwill and other intangibles assets
Number of operating segments | segment	3
Total goodwill	$ 311,536		$ 267,685
First Community
Goodwill and other intangibles assets
Total goodwill				$ 115,982
Other intangible assets				13,979
Banc Ed
Goodwill and other intangibles assets
Total goodwill		$ 41,437
Other intangible assets		$ 32,617
Pulaski
Goodwill and other intangibles assets
Total goodwill				$ 116,000
Mid Illinois
Goodwill and other intangibles assets
Changes in the carrying amount of goodwill	1,700
Banking
Goodwill and other intangibles assets
Total goodwill	288,436		246,999
Banking | Banc Ed
Goodwill and other intangibles assets
Total goodwill	41,400
Other intangible assets	27,200
Remittance Processing
Goodwill and other intangibles assets
Total goodwill	8,992		8,992
Wealth Management
Goodwill and other intangibles assets
Total goodwill	14,108		$ 11,694
Other intangible assets	5,400
Wealth Management | IST
Goodwill and other intangibles assets
Total goodwill	2,400
Other intangible assets	$ 5,700